2024: Add `macro_rules` `expr` and `expr_2021`

[ehuss]

Draft pending stabilization of 2024.

Tracing issue: https://github.com/rust-lang/rust/issues/123742

[mattheww]

Does macro-ambiguity.md need updating for this sort of change?

Eg should this line:

  * FOLLOW(expr) = FOLLOW(stmt) =  {`=>`, `,`, `;`}`.

now list FOLLOW(expr_2021) too?

[ehuss]

@mattheww Are you suggesting it should be this?

-  * FOLLOW(expr) = FOLLOW(stmt) =  {`=>`, `,`, `;`}`.
+  * FOLLOW(expr) = FOLLOW(expr_2021) = FOLLOW(stmt) =  {`=>`, `,`, `;`}`.

[mattheww]

Yes, I think that's how it works.

[ehuss]

Thanks! Yea, that makes sense to me.

[ehuss]

Merging now that https://github.com/rust-lang/rust/pull/129972 has stabilized.