feat: typescript types

[buschtoens]

This is a shot at converting ember-concurrency-decorators to TypeScript using ember-cli-typescript, so that we get auto-generated .d.ts files for using the decorators.

This is unfortunately not perfect, because:

• while we do get IntelliSense for the decorator arguments, we can't get it for the actual created TaskProperty, because TypeScript doesn't yet allow decorators to change the type signature: Request: Class Decorator Mutation microsoft/TypeScript#4881 (comment)
• there are not types for @ember-decorators/utils
• there are no types for ember-concurrency: Add typedefinitions ember-concurrency#209

This is why all of this is kinda ad-hoc and we need to // @ts-ignore in the tests.

I also need to clean up a little. Everything looks even more messy now and my makeshift types for ember-concurrency suck. 😄

[buschtoens]

Rebased onto latest master. But the generated types suck. 😄
I'll revisit it.

[buschtoens]

Since stage 2 decorators are miles away from TS decorators API-wise, it does not make sense any more to convert this to TypeScript just yet. When decorators are properly supported in TS, I'll revisit this.

In the meantime, I've already provided as manually created index.d.ts.