feat: add C# 12 collection expression support

[slang25]

Summary

• Implements grammar rules for collection expressions ([e1, e2, ..e3]) and spread elements (..)
• Resolves syntax conflicts with list patterns and range expressions through precedence rules and GLR declarations
• Adds element_binding_expression handling to initializer expressions to preserve dictionary/indexer initialization syntax
• All 164 existing tests pass, plus 9 new collection expression test cases

Test Coverage

Collection expressions tested for: empty literals, simple elements, spread operators, trailing commas, nested collections, method calls in spreads, use as arguments, and conditional spreads with parentheses.

🤖 Generated with Claude Code

[guillaume86]

I independently worked on the same feature in #414 and validated this PR against the pathological case from my issue comment: (int[])[Constants.Region.US, Constants.Region.EU, ...] inside a foreach was causing greedy error recovery that swallowed subsequent method declarations.

Confirmed this PR fully fixes it — all methods parse correctly with zero ERROR nodes. Ternary + collection (condition ? [x] : [y]) also works.

The _initializer_index_assignment approach to preserve element_binding_expression in initializer contexts is cleaner than what I had. 👍

Note: the CI parse-examples step will likely fail due to pre-existing issues — PowerToys and NUnit repos have been updated since the last master CI run, introducing files at new paths that aren't in the exclude list yet. See the exclude list update in my (now-closed) #414 for the specific files that need adding.