Auto merge of #123865 - eholk:expr_2021, r=fmease

Update `expr` matcher for Edition 2024 and add `expr_2021` nonterminal

This commit adds a new nonterminal `expr_2021` in macro patterns, and `expr_fragment_specifier_2024` feature flag.

This change also updates `expr` so that on Edition 2024 it will also match `const { ... }` blocks, while `expr_2021` preserves the current behavior of `expr`, matching expressions without `const` blocks.

Joint work with `@vincenzopalazzo.`

Issue #123742

Author: bors