Document TypeScript binder and symbols system

[Copilot]

This PR adds comprehensive technical documentation explaining how TypeScript's binder and symbols work internally. The documentation covers the core concepts needed to understand symbol resolution and name lookup behavior.

What's Added

Technical Documentation in .github/copilot-library.md

The new documentation explains:

• Symbols: TypeScript's internal representation of declarations with their key properties (flags, escapedName, declarations, members, exports)
• SymbolFlags: Bitmask enum categorizing declaration types (Value, Type, Namespace, etc.) and their merging rules
• SymbolTable: Map-based storage system for organizing symbols by escaped names
• Binder Process: How symbols are created during AST traversal, including conflict resolution and merging logic
• Name Resolution Algorithm: Detailed explanation of how resolveName works with context-sensitive scope chain walking

Key Example Explained

The documentation thoroughly explains the classic name shadowing example:

type x = number;           // Creates symbol: flags=TypeAlias, name="x"
function fn(x: string) {   // Creates symbol: flags=FunctionScopedVariable, name="x"
  let y: x = x;            // Two different lookups happen here
}

Shows how:

• Type position x resolves to the global type alias (number)
• Value position x resolves to the function parameter (string)
• Context (Type vs Value meaning) and scope hierarchy determine resolution

Validation Test Case

Added tests/cases/compiler/symbolsDocumentationTest.ts demonstrating:

• Symbol resolution across different scopes and contexts
• Interface and namespace merging behavior
• Parameter shadowing of global symbols
• Block-scoped variable shadowing

The test baselines clearly show how the same name resolves to different symbols based on context and scope.

Additional Improvements

• Added question to .github/copilot-questions.md about complex symbol merging scenarios
• All code follows TypeScript project formatting and linting standards

This documentation enables developers to understand the finer points of how resolveName works and debug complex symbol resolution issues.

Fixes #16.

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