This commit also adds means to manually disable warnings
in `code` tags where it's a false positive with the new
`skip-lint` attribute.
Warnings are now enabled on CI to prevent future errors.
We don't use that info for anything, and it generates unnecessary diffs
every time we bump the minor version (and CI failures if we forget to
sync some files from opt-in modules (mono, text_server_fb).
This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
