Move $$crate into its own feature gate

[nik-rev]

I'd like to stabilize $$ meta-variable expression escapes.

$$crate is the only blocker, and we're not 100% sure how to handle it, so it should be stabilized separately.

I am therefore creating a new incomplete feature gate macro_metavar_expr_dollar_dollar_crate which prevents people from using $$crate, and sending a stabilization PR for $$ after this one gets merged.

Tracking Issue: #155111

History:

• Stabilization of $$ was attempted in: Stabilize $$ in Rust 1.63.0 #95860.
• It was later reverted: Revert "Stabilize $$ in Rust 1.63.0" #99435
• The revert happened because we don't know how we should handle $$crate:
  • $$crate unexpectedly fails to defer which crate $crate refers to #99035
  • What exactly is $$crate's resolution span/crate? #151175
• A PR was made to forbid $$crate: Forbid $$crate #99193
• Which was then closed because the author lost motivation: Forbid $$crate #99193 (comment)

cc @rust-lang/lang (Responsible team)
cc @petrochenkov (Reviewed most of the implementation)
cc @CAD97 (Created the issue)
cc @c410-f3r (Author of the original stabilization PR back in 1.63, and the forbid-$$crate PR)
cc @Mark-Simulacrum (Nominated the issue)

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