Refactor the codebase to TypeScript

[nachtjasmin]

Because I got annoyed by the polyfill desaster and the different ways of handling browser-specific code, I've decided to begin the refactoring to TypeScript.

At least, TypeScript has interfaces and although we could work without it and keep a pure JS codebase, it's probably the cleanest way of doing such things. Instead of relying on the polyfill, we now can use dependency injection and therefore don't need to mock module imports.