Add support for mscz thubmnails

[Joshua-Beatty]

Adds support for mscz thumbnails. Closes #363

Some of the older mscz I found don't have a thumbnail file so I default to the regular image.

This zip file has some example mscz files, 2 with thumbnails, one without.
MSCZ Examples.zip

[CyanVoxel]

New thumbnail work is currently getting pointed to the thumbnails branch, which includes a host of new refactors for how extensions are handled and how the render methods are structured. To follow the pattern over there, this code would be put into it's own method while the MSCZ extension would be included in a new MediaType, MediaTypeCategory, etc. (Note: Not sure if this might count as an OpenDocument format?) There's a lot of precedent for the patterns over there to help guide you, but if you have any questions feel free to ask. Also while I haven't documented this anywhere yet, I'm trying to keep the MediaType lists and these render methods in alphabetical order.

Besides that, I appreciate the work on the feature! Cool to see new thumbnail support, especially when it just relies on the Python Standard Library!

[CyanVoxel]

I'm moved this PR from the deprecated thumbnails branch to Alpha-v9.4, which is the branch that thumbnails was eventually merged into. However the v9.4.x releases are now feature frozen and are now solely reserved for bugfixes. This means that after the changes from thumbnails are ported to main that I'll once again be moving the target for this PR over to there as a v9.5 feature.

Sorry for the disruption, and thank you for your patience.