Fix var keyword tokenization in tuple deconstruction at top level

[Copilot]

Fixes an issue where var keywords were not being correctly tokenized in tuple deconstruction patterns at the top level (outside of methods), causing them to appear as regular variables instead of type keywords.

Problem

When using tuple deconstruction with var keywords at the top level:

(int _, var _) = (1, 2);
(var _, var _, var _) = ('a', 'b', 'c');

The var keywords were being tokenized as variable.other.readwrite.cs (regular variables) instead of storage.type.var.cs (type keyword), preventing them from being highlighted correctly in the editor.

Root Cause

The local-tuple-var-deconstruction pattern only matched var (x, y) = ... syntax but not (var x, var y) = ... syntax at the statement level. When at the top level, the parenthesized expression pattern would match first and treat var as an identifier.

Solution

Added a new local-tuple-declaration-deconstruction pattern to the grammar that:

• Matches tuple deconstruction with type/var declarations: (type/var id, type/var id) = ...
• Uses tuple-declaration-deconstruction-element-list to properly handle var keywords
• Is included in local-declaration so it's checked before general expression patterns

Changes

• Grammar fix: Added local-tuple-declaration-deconstruction pattern in src/csharp.tmLanguage.yml
• Test coverage: Added 4 test cases covering both Input.InMethod and Input.FromText scenarios for:
  • Mixed type and var: (int _, var _) = (1, 2);
  • Multiple vars with discards: (var _, var _, var _) = ('a', 'b', 'c');

All 897 tests pass. The var keyword is now correctly tokenized as storage.type.var.cs at both method and top levels.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>var in tuple deconstruction</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?
1.86.1
What color theme are you using?
dark plus

Repro

(int _, var _) = (1, 2);
(var _, var _, var _) = ('a', 'b', 'c');

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

<agent_instructions>From the example, it appears that var is not being classified in tuple deconstruction</agent_instructions>

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

• Fixes var in tuple deconstruction #316

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