WIP fix(types): ensure correct oldValue typing based on lazy option

[colinscz]

• adjust test cases and add new ones
• correct vm.$watch calls, which are currently not matching anymore

No overload matches this call.
  Overload 1 of 2, '(expOrFn: string, callback: (this: CombinedVueInstance<Vue, object, object, object, Record<never, any>>, n: any, o: any) => void, options?: WatchOptions | undefined): () => void', gave the following error.
    Das Argument vom Typ "() => void" kann dem Parameter vom Typ "string" nicht zugewiesen werden.
  Overload 2 of 2, '(expOrFn: (this: CombinedVueInstance<Vue, object, object, object, Record<never, any>>) => void, callback: (this: CombinedVueInstance<Vue, object, object, object, Record<...>>, n: void, o: void) => void, options?: WatchOptions | undefined): () => void', gave the following error.

• corrected typos
• adjust method headers and add typings

#253

[LinusBorg]

This will have a merge conflict with #275 which should be merged next to ensure we keep up with the RFC.

Would you be up to adjust your PR once #275 has been merged?

[colinscz]

@LinusBorg sure I can do that after #275 has been merged.

[colinscz]

@LinusBorg I just saw that I took the wrong branch (dev) for this pull request, I will close this one and open a new one, which is based of master. That PR will include the discussed changes from #275 and my work.