refactor: Migrate Blockly.onKeyDown out of blockly.js

[maribethb]

The basics

• I branched from goog_module
• My pull request is against goog_module
• My code follows the style guide
• My code is presented in the form suggested in the module
  conversion guide
• I have run npm test.

The details

Resolves

Part of #5026 and #5208

Proposed Changes

Moves Blockly.onKeyDown to a local method on inject.js. This method was @package and only called from Blockly.inject to handle all keydown events on the page. This probably isn't something external developers would have been calling since we register it for them (and they shouldn't be since it's package) so I did not create an alias for it. Also, this is not marked as a breaking change since this should not affect code outside of Blockly.

Since the function is now not exported from its module, it is not accessible outside of this file. For the unit tests, I changed the tests to dispatch an event on the document instead of calling the event handler directly.

Additional Information

In order to get the unit tests to work with dispatchEvent, I had to create an actual KeyboardEvent object instead of the fake implementation previously used in the test_helpers. You can't just change the target type in the real event like this, so I removed that part. The only time it was ever used was for the "Not called when focus is an HTML input test" but that test was actually already broken. The event was created by passing in this.event as the first argument, which should be a keycode. So the keycode was not ESC so hideChaff was not called for that reason, and not because of the input. I've fixed this by dispatching the event on an actual html input, which correctly does not call the keyboard shortcut.

I also had to change the shortcut_registry tests to use an actual keycode instead of a random string, otherwise the event would not be created correctly.