Add unit testing

[erikpena]

• Updated to use Karma
• Using webpack builds transpiled by esbuild

[mmcc]

Looooove these tests. Karma is...a lot. Been a while since I've seen the Karma hello world. Should we consider using Jest here from the start? We switched the Dashboard client over a couple of years ago, although I know things like Mux Embed are still Karma-ified.

This is also my fault for not paying enough attention to what was going on when Buildkite was getting added, but let's use Github Actions here. We've gone that route for all our other open source, and since this will hopefully go that direction let's move this there as well.

[dylanjha]

I can vouch for Jest which seems to be tried-and-true now at this point and I've had a good experience. It's basically 0 config to get it going which is really nice. Unless we get something from karma that jest doesn't give us (testing in actual browsers, maybe?) I would lean that way.

Since this is open source, I would 👍 to Github Actions as well (only Mux folks can access our buildkite instance so outside contributors wouldn't have access to debug failing tests). It also should be less painful to set up.

[mmcc]

Closing this one because I think it ended up getting replaced by #7