Rust syntax if match

[hanbings-agent]

Introduces if <expr> matches <pat> as syntactic sugar for if let <pat> = <expr>.

This change provides a more fluent and natural syntax for conditional pattern matching, offering an alternative to if let.

The new syntax if <expr> matches <pat> is parsed into the same ExprKind::Let(<pat>, <expr>, ..) AST node as if let <pat> = <expr>, ensuring identical semantics and no breaking changes. This special parsing for matches only occurs within if/while conditions (when Restrictions::ALLOW_LET is active) to avoid conflicts with matches used as a regular identifier.

A UI test has been added to cover the new syntax and confirm that matches as an identifier remains valid in other contexts.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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