Add render-menu function.

[jmercouris]

Description

• This adds a function render-menu. This function can be used to generate menus for internal modes.

It also a fixes an issue where editor buffers could not have execute Lisp URLs. It fixes this by classing urls starting with editor as internal URLs.

Fixes #3162

Checklist:

• [X] Git branch state is mergable.
• [X] Changelog is up to date (via a separate commit).
• [X] New dependencies are accounted for.
• [X] Documentation is up to date.
• [X] Compilation and tests ((asdf:test-system :nyxt/gi-gtk))
  • No new compilation warnings.
  • Tests are sufficient.

[jmercouris]

Obligatory screenshot: