Add VariableParser and VariableContext

[N1ebieski]

This PR adds support for VariableParser and VariableContext.

For example, currently, this command:

php-parser detect "<?php

Gate::authorize('edit', \$user);" --debug

returns:

================================================================================
================================================================================
                              STARTING TO PARSE
================================================================================
================================================================================

Microsoft\PhpParser\Node\SourceFileNode <?php Gate::authorize('edit', $user);
+ Context: App\Contexts\Base
* Parsing: App\Parsers\SourceFileNodeParser

 Microsoft\PhpParser\Node\Statement\InlineHtml <?php 
 + Context: App\Contexts\Base
 * Parsing: App\Parsers\InlineHtmlParser

 Microsoft\PhpParser\Node\Statement\ExpressionStatement Gate::authorize('edit', $user);
 + Context: App\Contexts\Base
 * Parsing: App\Parsers\ExpressionStatementParser

  Microsoft\PhpParser\Node\Expression\CallExpression Gate::authorize('edit', $user)
  + Context: App\Contexts\MethodCall
  * Parsing: App\Parsers\CallExpressionParser

   Microsoft\PhpParser\Node\Expression\ScopedPropertyAccessExpression Gate::authorize
   + Context: App\Contexts\MethodCall
   * Parsing: App\Parsers\ScopedPropertyAccessExpressionParser

    Microsoft\PhpParser\Node\QualifiedName Gate
   Microsoft\PhpParser\Node\DelimitedList\ArgumentExpressionList 'edit', $user
   + Context: App\Contexts\MethodCall
   * Parsing: App\Parsers\ArgumentExpressionListParser

    Microsoft\PhpParser\Node\Expression\ArgumentExpression 'edit'
    + Context: App\Contexts\Argument
    * Parsing: App\Parsers\ArgumentExpressionParser

     Microsoft\PhpParser\Node\StringLiteral 'edit'
     + Context: App\Contexts\StringValue
     * Parsing: App\Parsers\StringLiteralParser

    Microsoft\PhpParser\Node\Expression\ArgumentExpression $user
    + Context: App\Contexts\Argument
    * Parsing: App\Parsers\ArgumentExpressionParser

     Microsoft\PhpParser\Node\Expression\Variable $user
[
    {
        "type": "methodCall",
        "methodName": "authorize",
        "className": "Gate",
        "arguments": {
            "type": "arguments",
            "autocompletingIndex": 2,
            "children": [
                {
                    "type": "argument",
                    "name": null,
                    "children": [
                        {
                            "type": "string",
                            "value": "edit",
                            "start": {
                                "line": 2,
                                "column": 16
                            },
                            "end": {
                                "line": 2,
                                "column": 20
                            }
                        }
                    ]
                },
                {
                    "type": "argument",
                    "name": null,
                    "children": []
                }
            ]
        }
    }
]

With VariableParser and VariableContext:

================================================================================
================================================================================
                              STARTING TO PARSE
================================================================================
================================================================================

Microsoft\PhpParser\Node\SourceFileNode <?php Gate::authorize('edit', $user);
+ Context: App\Contexts\Base
* Parsing: App\Parsers\SourceFileNodeParser

 Microsoft\PhpParser\Node\Statement\InlineHtml <?php 
 + Context: App\Contexts\Base
 * Parsing: App\Parsers\InlineHtmlParser

 Microsoft\PhpParser\Node\Statement\ExpressionStatement Gate::authorize('edit', $user);
 + Context: App\Contexts\Base
 * Parsing: App\Parsers\ExpressionStatementParser

  Microsoft\PhpParser\Node\Expression\CallExpression Gate::authorize('edit', $user)
  + Context: App\Contexts\MethodCall
  * Parsing: App\Parsers\CallExpressionParser

   Microsoft\PhpParser\Node\Expression\ScopedPropertyAccessExpression Gate::authorize
   + Context: App\Contexts\MethodCall
   * Parsing: App\Parsers\ScopedPropertyAccessExpressionParser

    Microsoft\PhpParser\Node\QualifiedName Gate
   Microsoft\PhpParser\Node\DelimitedList\ArgumentExpressionList 'edit', $user
   + Context: App\Contexts\MethodCall
   * Parsing: App\Parsers\ArgumentExpressionListParser

    Microsoft\PhpParser\Node\Expression\ArgumentExpression 'edit'
    + Context: App\Contexts\Argument
    * Parsing: App\Parsers\ArgumentExpressionParser

     Microsoft\PhpParser\Node\StringLiteral 'edit'
     + Context: App\Contexts\StringValue
     * Parsing: App\Parsers\StringLiteralParser

    Microsoft\PhpParser\Node\Expression\ArgumentExpression $user
    + Context: App\Contexts\Argument
    * Parsing: App\Parsers\ArgumentExpressionParser

     Microsoft\PhpParser\Node\Expression\Variable $user
     + Context: App\Contexts\Variable
     * Parsing: App\Parsers\VariableParser

[
    {
        "type": "methodCall",
        "methodName": "authorize",
        "className": "Gate",
        "arguments": {
            "type": "arguments",
            "autocompletingIndex": 2,
            "children": [
                {
                    "type": "argument",
                    "name": null,
                    "children": [
                        {
                            "type": "string",
                            "value": "edit",
                            "start": {
                                "line": 2,
                                "column": 16
                            },
                            "end": {
                                "line": 2,
                                "column": 20
                            }
                        }
                    ]
                },
                {
                    "type": "argument",
                    "name": null,
                    "children": [
                        {
                            "type": "variable",
                            "name": "user",
                            "start": {
                                "line": 2,
                                "column": 24
                            },
                            "end": {
                                "line": 2,
                                "column": 29
                            }
                        }
                    ]
                }
            ]
        }
    }
]

In the future, this can be extended to include reading docblocks to retrieve value.