Addresses part of https://github.com/coder/registry/issues/194
## Description
This PR beefs up the validation for the validation logic that we already
had in place. This PR does not include adding validation for templates
(which will be addressed in a second PR).
### Changes made
- Added logic to reject unknown frontmatter fields for modules and
contributor profile README files
- Added logic to handle frontmatter fields that were previously missed
in validation steps (GitHub username for contributors and Operating
Systems for modules)
- Updated a few comments (added some new comments, formatted existing
comments to meet 100-column width)
### Type of Change
- [ ] New module
- [x] Bug fix
- [ ] Feature/enhancement
- [ ] Documentation
- [x] Other
## Testing & Validation
- [x] Tests pass (`bun test`)
- [x] Code formatted (`bun run fmt`)
- [x] Changes tested locally
fd074a5643