Add a linter/fix script for mdn_urls

[Elchi3]

This PR uses the mdn/content inventory and mdn/content's _redirect.txt file to update mdn_urls automatically.

• It removes 404 mdn_urls
• It replace redirect urls with the new url per the redirect file
• It replaces wrong casing
• It adds new mdn_urls when pages have been found existing at conventional places.

It works with the https://www.npmjs.com/package/@ddbeck/mdn-content-inventory package. Whenever dependabot updates this dependency it might be that this lint fails, maintainers can then look at the failures and run npm run fix to update mdn_urls automatically and, if needed, make manual changes.

The initial update to mdn_urls (that will make this PR pass CI) is #23481 and so this PR is blocked by having #23481 merged.

[Josh-Cena]

See #23481 (review)

[Elchi3]

@Josh-Cena

FWIW this seems slightly inconvenient in the context of fixing MDN gaps, since every new doc now also requires a BCD update, but I think it's generally an improvement, and it'll probably take us a long while to fix all MDN gaps...

Thanks for your feedback. What you describe is already happening and leads to lots of PRs and review work which IMO can be automated. My intent with this PR is to actually prevent PRs like #23499 (and see tons of them at: https://github.com/mdn/browser-compat-data/pulls?q=is%3Apr+mdn_url+is%3Aclosed). The idea is that we add mdn_urls automatically whenever dependabot updates the mdn-content-inventory package (which will release frequently).