fix(toml): Update frontmatter parser for RFC 3503 (#14792)

### What does this PR try to resolve?

We removed rejected syntax in #13861 haven't update frontmatter parsing
to what [RFC
3503](https://rust-lang.github.io/rfcs/3503-frontmatter.html#reference-level-explanation)
says.

This adds tests and fixes various cases to ensure we correctly detect
the frontmatter and the infostring.

This is part of #12207

### How should we test and review this PR?

### Additional information