Add themecheck to validate color codes in themes, ignore current known errors.

[askreet]

Looking for some feedback on approach here while thinking about #5709.

This patch:

• Adds a first-class concept of warnings to theme parsing, which currently catches all the hexcode parse errors from Invalid hexcodes in several themes emacs, monokai_aqua, papercolor-light (Should be checked in CI at theme PRs?) #5709.
• Updates themelint with a known issues list, so it can be enabled in CI and prevent new errors.

If this is interesting to folks the next steps would be:

• Adding it to CI.
• Plumbing the warnings from other bits of theme.rs up through Loader::load, too, and adding any known issues.

Notes:

• I moved the "//TODO: alert the user..." comment up to a higher level in the code as I think that goal relates more to sending an event to the UI, which I haven't looked into.

[the-mikedavis]

A check in the CI that fails when there are missing hexcodes would be a good addition but we don't want to add themelint to the CI. (Actually we might remove it in the long-run #5534 (comment).) If the palette check can be separated from the existing themelint xtask I think this would be a good change 👍

[askreet]

Great context, thanks @the-mikedavis. I'll separate these checks out.

[askreet]

Pushed up some changes to isolate a new check called themecheck that has been added to CI. Open to suggestions on the naming/scoping of the checks. @the-mikedavis

[askreet]

@archseer Can I get a quick 👍🏼 / 👎🏼 on whether this is interesting for the project? No pressure on merge timeline, but if you give me a 👍🏼 I will keep it up to date with master periodically, otherwise I'll close it out.