Rollup merge of #149823 - epage:f, r=Kivooeo

fix(parser): Disallow CR in frontmatter

T-lang came back on the stabilization PR (rust-lang/rust#148051) asking for CR to be disallowed
to leave room for all stray CRs to be rejected in the future.
At that point, the test can remain but the implementation can be
removed.

If that plan does not go through, we'll need to re-evaluate
- whether this is more lint-like and should defer to the calling tool
  that is managing the frontmatter
- how much Rust should treat the frontmatter as Rust and apply the same
  grammar restrictions of "no stray CR" (like raw string literals)

Part of rust-lang/rust#136889

Author: Zalathar