Explicit ignored binding syntax

[hanbings-agent]

This PR introduces the ignored keyword as an explicit placeholder for ignored bindings in pattern destructuring, behind the #[feature(explicit_ignored_bindings)] gate.

The motivation is to improve semantic clarity and explicitly express the intent of ignoring a binding, similar to how async fn, impl Trait, and dyn Trait enhance readability by making intent more explicit.

For example, let (a, ignored) = (1, 2); now explicitly states that the second element is intentionally ignored, behaving identically to let (a, _) = (1, 2);.

The ignored keyword is implemented as a weak keyword and parses into PatKind::Wild.

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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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