Enforce that raw lifetimes must be valid raw identifiers
Make sure that the identifier part of a raw lifetime is a valid raw identifier. This precludes `'r#_` and all module segment paths for now.
I don't believe this is compelling to support. This was raised by `@ehuss` in https://github.com/rust-lang/reference/pull/1603#discussion_r1822726753 (well, specifically the `'r#_` case), but I don't see why we shouldn't just make it consistent with raw identifiers.
