Update link valiation to include known protocols

[zkamvar]

This updates the $validate_links() method to now include an allowed list of protocols and will error if the protocol is not within the list. I have derived the list of protocols from Wordpress: https://developer.wordpress.org/reference/functions/wp_allowed_protocols/#return

Links with protocols not within our allow list will be flagged:

      link-test.md:45  [invalid protocol] (gttps) [needs HTTPS] gttps://example.com
      link-test.md:47  [invalid protocol] (bitcoin) [needs HTTPS] bitcoin:FAKE-EXAMPLE
      link-test.md:48  [invalid protocol] (javascript) [needs HTTPS] javascript:alert%28%27JavaScript%20Link!%27%29

I have also updated the error messages and documentation around link validation.

This will fix #107 and make sure that fishtree-attempt/instructor-training#5 actually throws a warning