Add MouseEvent for tests

[justinwaite]

Stencil version:

 @stencil/core@1.3.1

I'm submitting a:

[x] feature request

Current behavior:
Currently MouseEvent is not accessible during jest tests.

Expected behavior:
The user should be able to mock MouseEvent events for their unit tests.

Steps to reproduce:
Try using MouseEvent in a unit test for a component and you will receive a MouseEvent is not defined error.

Other information:

See #1581 when the KeyboardEvent was added.

I plan on creating a PR for this to mirror the work done for KeyboardEvent.