refactor(frontmatter): Pull out as a dedicated mod (#15914)

### What does this PR try to resolve?

This sets up the frontmatter logic as being more standalone in prep for
- Adding a custom error type that tracks spans for a more rustc-like
error experience
- Potentially moving this out into a dedicated package

### How to test and review this PR?

### Notes

Had this sitting around for a while but haven't been able to finish the
work that builds on it so wanted this merged to reduce conflicts.

Author: ehuss