Copy over changes from other repo (mostly a continuation of validator fixes)

[cooljeanius]

This PR contains the following:

• new linting workflow
• recompression of images
• start of work on some lua code
• validator fixes
• updated README

See the following issues:

• partially inspired by issue #7
• partially addresses a follow-up comment I made in issue #32
• lets us check off checkbox 2 in issue #33
• using the validator had a bit of the same effect that attempting to address issue #57 would have had
• gets started with issue #73 (only a bit, though; I'm holding back on some other parts I've written for it until they're more submittable)
• adds some FIXMEs before lines that the validator pointed out that I think might be causing issue #81
• Closes #144 (at least I think that ought to be enough for now)

[cooljeanius]

I assume you playtested these changes? As in nothing breaks when you load the scenarios.

I only tested the first few scenarios, since the --validate-addon flag will check the entire campaign upon just loading the first scenario... trying to switch to some other scenarios with the :cl in-game debug command, though, reveals a bug that was probably already there previously that I'd never noticed before due to not having tried switching levels this way: when skipping from S04 ("Battle of Barnon") to S05 ("The Swamp Things") I get:

20240103 01:29:55 info wml: A [recall] tag with the following content failed:
show = no
side = 1
x = 1
y = 1
20240103 01:29:57 error wml: [unstore_unit]: variable 'ano_reme_stored' doesn't contain unit data

I'll open a separate issue about that. Note that I'm planning several more PRs and another full playthrough before the next release, so if anything else comes up, I can fix it later...

[cooljeanius]

I'll open a separate issue about that.

...ok, I opened #165 for it.