Fix url validation errors

[talldan]

Description

When adding href validation in #11286 I made a few errors, which this PR addresses

1. Fragments in URLs were not being checked correctly, any existence of a fragment would make a URL fail the validation 🤦‍♂️ . This was happening due to the wrong value being passed to isValidFragment (the entire href was being passed instead of just the fragment).
2. Because of the issue above, tests were also returning a false positive when validating incorrect fragments with two hashes like '#test#test'. This was happening because getFragment only captured from the last occurrence of a hash, now it captures from the first.
3. Validation of HTTP urls didn't catch issues like the incorrect number of forward slashes following the protocol. I've added a special case for HTTP urls.
4. isValidProtocol considered a protocol with trailing forward slashes as valid. I realised this didn't match what getProtocol considered a protocol (characters up to and including the colon, e.g. 'http:'), so I've made it stricter.

How has this been tested?

• Added test cases to unit tests
• Manual testing

1. Try adding one of the following URLs as a link:

• https:/test.com
• https:///test.com
• https:test.com
• https://test.com#test#test

2. Expect the validation to fail (the link should be displayed in red text).
3. Try adding the following URL as a link:

• https://test.com#test

4. Expect the validation to succeed (the link should be displayed normally).

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[noisysocks]

👍 looks good!

[noisysocks]

😅