Not being able to set a keyCode for a KeyboardEvent

[grassator]

Describe the bug
Right now KeyboardEvent does not allow to set a keyCode.

To Reproduce

const e = new KeyboardEvent('keydown', { keyCode: 40 });
console.log(e.keyCode); // undefined, expected 40

Expected behavior
I would expect keyCode to be propagated from the event init object to the event itself.

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The change is pretty trivial and I am happy to submit a PR if required