Add typings

[trustedtomato]

As the project already kinda uses JSDocs for typing, would be nice if we added proper typings with JSDocs to the whole project to reduce the amount of, for example, null errors. Could be done together with #75, since both issues require changes in more-or-less all files, so merging them in parallel would be a little painful.

[Mr0grog]

Sure, this would be good, but please do not combine with #75. Also, I’d say #75 is certainly higher priority here, if you’re trying to figure out how to sequence things.