Rewrite on_unimplemented format string parser.
This PR rewrites the format string parser for `rustc_on_unimplemented` and `diagnostic::on_unimplemented`. I plan on moving this code (and more) into the new attribute parsing system soon and wanted to PR it separately.
This PR introduces some minor differences though:
- `rustc_on_unimplemented` on trait *implementations* is no longer checked/used - this is actually never used (outside of some tests) so I plan on removing it in the future.
- for `rustc_on_unimplemented`, it introduces the `{This}` argument in favor of `{ThisTraitname}` (to be removed later). It'll be easier to parse.
- for `rustc_on_unimplemented`, `Self` can now consistently be used as a filter, rather than just `_Self`. It used to not match correctly on for example `Self = "[{integer}]"`
- Some error messages now have better spans.
Fixes https://github.com/rust-lang/rust/issues/130627
aad59a30de