Feature request: parse new `let_else` syntax

rust-analyzer currently can't parse the new refutable assignment syntax gated by the `let_else` unstable feature. It reports "expected SEMICOLON" and "expected expression".

rustc feature merge request: https://github.com/rust-lang/rust/pull/87688
rustc tracking issue: https://github.com/rust-lang/rust/issues/87335