Add CI to the project

[derekwyatt]

We have some Ruby scripts in the repo that can perform some testing for us, but there's not a ton of things that it covers. Let's discuss the hurdles to be leaped in this issue and design an approach:

• What's the state-of-the-art for testing these sort of plugins for Vim?
• Can we automate the testing of the syntax rendering?
• Is there a particular service / GitHub action that works "the best"?
• Is there a way for it to be run locally as well? (Creating a PR just to run the tests is annoying)
• Will the errors be clear to the developer so they can easily understand what needs fixing?
• What's the appropriate minimum to test? We should avoid testing "too much" and venture into the world of flakey tests.

Anything else?

[derekwyatt]

@ches and anyone else... let's get some expert advice on this one :)

[benknoble]

I would recommend taking a look at some of ALE's CI infrastructure—I don't agree with all the style guidelines, but they have a very comprehensive test suite.