Add a script to find missing mimetypes icons.

[achadwick]

While working on #3904, I found myself wanting a script for discovering MIME types that don't have icons or symlinks yet. I thought I'd make one. This script scans the cached bits of the shared-mime-info database on the local device, and checks whether icons exist in the Papirus tree for those.

The idea is that only devs will use it; it's not part of any workflow, since it produces rather a lot of false positive output. By default, it flags anything defaulting to <type>-x-generic as a potential omission, after considering 1) named icons for the mimetype, 2) fallback icons for the exact mimetype string, and 3) named generic icons for the mimetype. Quite often, that's fine; at other times it's an indication that Papirus needs a more specific icon for the mimetype.

Ref:

• https://specifications.freedesktop.org/shared-mime-info-spec/latest/ (defines the contents of the database files)
• https://specifications.freedesktop.org/basedir-spec/latest/ (contains a partial implementation)
• https://specifications.freedesktop.org/icon-naming-spec/latest/#guidelines (don't chop on - for mimetypes)

Leaving this open for review for a bit.

[achadwick]

First question is: is this helper script in the right folder? 😁