Support macro metavar expressions

[Veykril]

A subset is being stabilized soon rust-lang/rust#95860
RFC https://github.com/rust-lang/rfcs/blob/master/text/3086-macro-metavar-expr.md#dollar-dollar

[TimNN]

At least for $$ the implementation seems to be pretty straight forward. If there's no-one else currently working on this, I'd like to take a stab at properly implementing the $$ support. (I still need to verify that the current behavior matches what rustc actually does, and especially if multiple levels of nesting behave correctly).

[Veykril]

Go ahead!