Export CommConstants HostObject to JavaScript

Summary: This differential exports CommValidationModule to JavaScript.

Test Plan:
I decided to call commConstants.NATIVE_MESSAGE_SPECS in sqlite-data-handler.js for testing purposes. So the test plan is:

1. Build the app
2. Apply this diff: https://gist.github.com/marcinwasowicz/da35ee527660a6a020daf88ddae61d24
3. Open the app. Ensure the warning message like this is visible:

Object {
  "message": "C++ MessageSpec implementation missing for message types: ADD_MEMBERS, REMOVE_MEMBERS, LEAVE_THREAD, JOIN_THREAD, TOGGLE_PIN",
}

Reviewers: bartek, atul

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8071