Convert to TypeScript

[buschtoens]

Closes #14.

[josemarluedke]

In order to allow the following call without type errors, the args needs to be optional:

  @use user = new FetchData(() => [
    `www.example.com/users/${this.args.userId}`
  ]);

Suggested change

	constructor(ownerOrThunk: Owner | Thunk, args: Args) {
	constructor(ownerOrThunk: Owner | Thunk, arg?s: Args) {