[w3c/pointerlock] movementX/Y to be double instead of long (#24)

Following [the extension](https://drafts.csswg.org/cssom-view/#extensions-to-the-mouseevent-interface) that changes screenX/Y type to double I'd suggest we also change the type of movementX/Y to double. @scheib do you imagine any scenario that this might break stuff for any developers?

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