Add support for typevar attribute target specifier

[Copilot]

The typevar attribute target specifier (C# 13) was not recognized, causing it to be incorrectly tokenized as separate type and var tokens.

Changes

• Grammar: Added typevar to the attribute-section pattern in src/csharp.tmLanguage.yml
  • Positioned before type in the regex alternation to prevent premature matching
• Tests: Added coverage for typevar attribute specifier

Example

class Foo<[typevar: MyAttribute] T, [typevar: MyAttribute] U>
{
    public static void F<[typevar: MyAttribute] X>() { }
}

Before: typevar highlighted as two separate tokens (type + var)
After: typevar highlighted as a single keyword attribute specifier

Original prompt

---

This section details on the original issue you should resolve

<issue_title>typevar keyword</issue_title>
<issue_description>## Details

What editor are you seeing the problem in? (e.g. Atom, Visual Studio Code, etc.)
vscode
What version of the editor are you using?
1.88.1
What color theme are you using?
dark-plus

Repro

[AttributeUsage(AttributeTargets.All)]
class MyAttributeAttribute : Attribute { }

class Foo<[typevar: MyAttribute] T, [typevar: MyAttribute] U>
{
    public static void F<[typevar: MyAttribute] X, [typevar: MyAttribute] Y>() { }
}

Please provide a code example and (optionally) a screenshot demonstrating the problem.</issue_description>

<agent_instructions>From the example, it appears the typevar attribute target is not being classified.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes typevar keyword #320

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