Moving macro metavar expressions forward

[tgross35]

The feature macro_metavar_expr has been around since 2021 without any significant progress rust-lang/rust#83527. There is one unresolved question listed about hygiene, but this may have since been resolved (unclear).

It would be nice to get parts of this on the path to stabilization, even if all parts of the RFC are not yet ready. Stabilizing only ${ignore(ident)} could make sense as a first step since it is is useful, has a well-defined purpose, and shouldn't suffer from the possible hygiene problem.

This is also relevant because there are other metavar expressions that we would like to eventually move forward, such as concat in rust-lang/rust#118958 (comment).

Discussion thread: https://rust-lang.zulipchat.com/#narrow/stream/404510-wg-macros/topic/Metavar.20expressions

[tgross35]

@c410-f3r actually recently suggested merging all of the feature at rust-lang/rust#83527 (comment), so this is on the table too.