Depends on https://github.com/lightpanda-io/wpt/pull/77
Track click count so that we can correctly fire a double click. On click, if
the mousedown and mouseup targets differe, find the nearest common inclusive
ancestor. Our `dispatchMouse` now sets the correct buttons value.
Also, HTMLElement.click() should be a PointerEvent (not a MouseEvent)
Improves various /uievents/ WPT cases.