Link Checker fork per hl

The extension checks all links on a website and highlights them based on the following categories
- OK
- Redirect
- Fail

It is a fork of https://addons.mozilla.org/firefox/addon/simple-link-checker

Source Code: https://git.fsfe.org/hl/simple-link-checker

Changes:
- OK Links are highlighted in light blue instead of green to help people with red-green vision impairment
- Metadata was updated to work on current Firefox versions and resolve all errors, warnings and notices
- The code was made REUSE compliant for future use, reuse and proper attribution