Add expr_2021 nonterminal and feature flag

This commit adds a new nonterminal `expr_2021` in macro patterns, and
`expr_fragment_specifier_2024` feature flag. For now, `expr` and
`expr_2021` are treated the same, but in future PRs we will update
`expr` to match to new grammar.

Co-authored-by: Vincezo Palazzo <vincenzopalazzodev@gmail.com>

compiler/rustc_span/src/symbol.rs

@@ -782,6 +782,8 @@ symbols! {
         explicit_tail_calls,
         export_name,
         expr,
+        expr_2021,
+        expr_fragment_specifier_2024,
         extended_key_value_attributes,
         extended_varargs_abi_support,
         extern_absolute_paths,