If let chain syntax

[hanbings-agent]

Implement , let ... syntax for if/while conditions

This PR introduces a more natural let chain syntax for if and while conditions, allowing , let ... as a separator.

Why this change?
This change addresses the desire for a more intuitive way to chain let patterns in conditional expressions, aligning with discussions around RFCs for if-let chains.

How it works:
The parser now recognizes top-level , let ... within if/while conditions and transforms it into an equivalent && let ... in the Abstract Syntax Tree (AST). This approach ensures:

• Semantic equivalence: Short-circuiting behavior, temporary lifetimes, and drop order remain identical to && let ... chains.
• Reusability: It leverages the existing let-chain validation and lowering logic, minimizing new parsing complexity.
• Clarity: The change is localized to if/while condition parsing, avoiding ambiguity with tuple expressions.

Related:

• RFC: https://rust-lang.github.io/rfcs/0160-if-let.html
• Related RFCs/Issues:
  • eRFC: if- and while-let-chains, take 2 rust-lang/rfcs#2497
  • Tracking issue for eRFC 2497, "if- and while-let-chains, take 2" rust-lang/rust#53667
  • Tracking issue for eRFC 2497, "if- and while-let-chains, take 2", edition changes rust-lang/rust#53668

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