Add syntax highlighting for fixed-size buffer declarations

[Copilot]

The fixed keyword in fixed-size buffer field declarations was not being highlighted, appearing in default text color instead of as a storage modifier like public and unsafe.

Changes

• Grammar: Added fixed-size-buffer-declaration pattern to handle the distinct syntax of fixed-size buffers (fixed <type> <name>[<size>])

  • Captures fixed as storage.modifier.fixed.cs
  • Parses buffer type, name, and size expression in brackets
  • Positioned before field-declaration to match correctly

• Tests: Added test token and three test cases covering:

  • Basic fixed-size buffer: public fixed byte Buffer[30]
  • With unsafe modifier: public unsafe fixed byte Buffer[30]
  • With const size reference: public unsafe fixed byte Buffer[length]

Example

unsafe struct B
{
    public fixed byte Buffer[30];  // 'fixed' now highlighted as storage modifier
}

struct C
{
    public const int length = 30;
    public unsafe fixed byte Buffer[length];  // works with const references
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>fixed-size buffer syntax</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?
1.88.0
What color theme are you using?
dark plus

Repro

unsafe struct B
{
    public fixed byte Buffer[30];
}
struct C
{
    public const int length = 30;
    public unsafe fixed byte Buffer[length];
}

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

<agent_instructions>From the example, it appears the fixed keyword is not being classified</agent_instructions>

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

• Fixes fixed-size buffer syntax #318

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