Offer types for `getters` on the Store instance

[rich-w-lee]

What problem does this feature solve?

The getters key of the Store instance has type any which is not useful. While type casting can be used, it's just a pain to have to do every single place you would want to reference a getter.

What does the proposed API look like?

One solution would be to offer a second (optional) generic type of the Store type that would default to any, and have that value be used for the type of the getters key.

export declare class Store<S, G = any> {
  constructor(options: StoreOptions<S>);

  readonly state: S;
  readonly getters: G;

  ...
}

I've tested the above solution and it works and should also be backwards compatible.

[kiaking]

Thanks for the feedback! I think we should come back to the typings once TS 4.1 is out. It might change quite a bit (in a good way). See #1831 for more details.

[jouni-kantola]

For codebases that use Options API it'd be nice to expose type parameters for getters, as it enables getter typings in components.

Currently, in vue-shim.d.ts we can expose the store to components with typeof, but that still leaves getters as any.