Generate a FocusEvent (not Event) for focus-related events

[davidrunger]

Currently on main, focus-related events (blur, focus, focusin, and focusout) create a JavaScript Event object.

However, as can be seen by clicking into and out of the input in this JSFiddle (tested in Firefox 130 and Chrome 128), those events should actually be JavaScript FocusEvent objects (not Event objects).

This change makes it so that Cuprite will generate FocusEvent objects (not Event objects) for focus-related events.