Add automated tests

[theY4Kman]

NOTE: this ticket may be more of a reminder than any solid plan

Brief

Language plugins are always a fair bit complex, and can easily grow in complexity — it's very easy for subtle (and not-so-subtle) bugs to be introduced and go unnoticed. Having an automated test suite is paramount to reducing these bugs, and staying sane, in general.

Details

Jetbrains provides a number of handy base test classes for verifying the behaviour of many language-related actions — things like testing refactoring, renaming, autocompletion, and I'd wager syntax-highlighting, too.

It may be wise to hook up GitHub Actions, Travis, Circle, or some other CI tool to automatically run tests on master and any PRs. To start on that route, we'll need a test suite of at least 1 test.