Refactor to TypeScript / Add typings

[agilgur5]

Because there's a high overlap between MobX devs and TS devs (due to MobX / MST being written in [some super complex] TS).

Could also use tsc or tsdx to output CJS builds and resolve #3, feeding two birds with one seed 😃

Could add typings separately to DefinitelyTyped first in incremental progress, but I'll probably end up doing it all at once (and then no need to confuse users to switch back / remove @types or keep @types backward compatible)