Add default semantic token rules for C# (Roslyn) token types

[tatsudo]

Roslyn reports an extended token-type legend (field, controlKeyword, recordClass, extensionMethod, ...) that the default rules did not cover, so those tokens were silently dropped and C# fell back to tree-sitter-only highlighting. Map them to existing theme styles.

Objective

Companion to #60015. This PR adds highlighting support for Roslyn's (C#) semantic token types, so C# code can be colored correctly instead of being limited to the standard LSP token set.

Zed's default semantic token rules only cover the ~24 standard LSP token types, and there's no catch-all rule. Roslyn reports a much larger, C#-specific legend (field, controlKeyword, recordClass, extensionMethod, ...), so any token whose type isn't one of those 24 is silently dropped — leaving C# fields, constants, control-flow keywords, records, etc. without semantic highlighting (a private field stays indistinguishable from a local variable).

Solution

Add default rules in default_semantic_token_rules.json mapping Roslyn's C# token types to existing theme styles (with fallback chains, in the existing // C# section).

Testing

• Added a test asserting the default rules parse and cover the added C# token types (guards against accidental removal).
• Verified manually on a C# project: fields, constants, control-flow keywords, records and extension methods now get semantic highlighting consistent with the theme.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Showcase

Click to view showcase

Before — Roslyn's C# token types aren't mapped, so fields/keywords fall back to tree-sitter (a field looks like a plain variable):

After — fields, constants, control-flow keywords and records get semantic colors:

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Release Notes:

Improved C# semantic highlighting by mapping Roslyn's token types (fields, constants, records, control-flow keywords, and more) to theme styles.

[MrSubidubi]

Thanks for this!

I see no reason to not just ship this with the C# extension, hence am gonna close this at this time. Feel free to inform me here should I be missing something. Thanks!