Add unit tests

[mre]

Since this plugin is slowly gaining popularity, I'd like to avoid breaking it with a bad release. Hence, I'd like to add some unit tests. I have no idea how to do that in the VSCode/Typescript ecosystem. Probably something like this from VSCodeVim: https://github.com/VSCodeVim/Vim/tree/master/test If anyone has an idea or can give me some pointers, I'd be very thankful.