Auto merge of #14878 - lowr:feat/metavar-expr-count, r=Veykril

Implement `${count()}` metavariable expression

This PR implements `${count()}` metavariable expression for MBE as described in [RFC 3086][rfc]. See [this section][sec-count] of the RFC for its semantics.

Additionally, this PR changes the type of depth parameter of `${index()}` from u32 to usize so as to match how [rustc parses it][index-usize].

Part of #11952
Fixes #14871

[rfc]: https://github.com/rust-lang/rfcs/pull/3086
[sec-count]: https://github.com/rust-lang/rfcs/blob/master/text/3086-macro-metavar-expr.md#count
[index-usize]:https://github.com/rust-lang/rust/blob/ddad0576caf8d0515ed453e04b468977c7d3dfc1/compiler/rustc_expand/src/mbe/metavar_expr.rs#L22
tree: 0677369a5a8fae58cf8ba0acb7f91018145ca11d
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