fix: Detect unsupported images used in static themes manifest

[rpl]

Fixes #2051

This PR adds the following 5 new validations rules (also added to the docs/rules.md file in this PR):

• errors:

  • MANIFEST_THEME_IMAGE_NOT_FOUND (Theme images missing)
  • MANIFEST_THEME_IMAGE_CORRUPTED (The linter is unable to read their mime types)
  • MANIFEST_THEME_IMAGE_WRONG_EXT (Theme images must have one of the supported file extensions)
  • MANIFEST_THEME_IMAGE_WRONG_MIME (Theme images mime type must be a supported format)

• warnings:

  • MANIFEST_THEME_IMAGE_MIME_MISMATCH (the format is supported, as well as the file extension, but the mime doesn't match the file extension the linter expects)

TODO list

• should we support webp as a valid static theme image?
• opted to currently disallow it as it is not yet officially supported based on the relevant MDN doc page
• double-check the additional tests are enough to bring back the code coverage to the current status why coveralls is not in the list of the checks? :-(

[mstriemer]

This looks good, lots of test cases.

A few comments, but nothing that would change the structure of this much.