jsaction.createKeyboardEvent does not work in Firefox 57.0+

[DrMarcII]

Reproduce: run tests in replay_test.js

Firefox has implemented KeyboardEvent.initKeyboardEvent, but it does not correctly set the keyCode or charCode.

Based on https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent, it looks like the preferred way to create these events is with the KeyboardEvent constructor (both initKeyEvent and initKeyboardEvent are deprecated). The most surprising part of this to me, is that MDN also says Gecko will not implement initKeyboardEvent, but it appears that something has changed.