widget-service-mocks: add API and entry point

[x1B]

To test LaxarJS v2 widgets (using laxar-mocks or custom means), access to mocks for the various widget injections would be very helpful. However, those are not really considered part of our public API.

To aid in testing, we should add a new API for widget testing with its own entry point, exposing only the relevant mocks.

A separate set of mocks should be exposed for tooling/adapter-testing (separate issue #350).

[x1B]

Implemented on master (v2.0.0-alpha.8).

Upgrade Information

A new entry point laxar-widget-service-mocks.js was added (available in the source root, and pre-bundled under dist/). It exposes factories for various mocks that can be used to simulate widget service injections for testing.

It is mainly used by laxar-mocks, but is also intended for testing libraries that work with LaxarJS widget services.