Add readonly modifier support for property getters

[Copilot]

The readonly keyword was not being highlighted when used as an accessor modifier for getters in properties and indexers (C# 8.0+).

struct M
{
    public int Counter
    {
        get => counter;
        readonly set { }  // readonly is not valid on setters
    }
    
    public int this[int i]
    {
        readonly get => counter;  // readonly was not highlighted
        set { }
    }
}

Changes

• Grammar: Added readonly as an optional modifier specifically for get accessors only (not set or init, as per C# language specification)
• Tests: Added coverage for readonly getters with both expression bodies and block bodies

The readonly modifier is now correctly restricted to getter accessors, matching C# 8.0+ language semantics for readonly instance members in structs.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Textmate] readonly getter and setter in struct</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?
1.85.1
What color theme are you using?
dark plus

Repro

struct M
{
    private int counter;
    public int Counter
    {
        get => counter;
        readonly set { }
    }
    public int this[int i]
    {
        readonly get => counter;
        set { }
    }
    public readonly void Method()
    {
        counter++;
    }
}

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

<agent_instructions>From the example, it appears that readonly is not being classified in readonly getters</agent_instructions>

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

• Fixes [Textmate] readonly getter and setter in struct #314

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