https://www.w3.org/TR/event-timing/#sec-performance-event-timing
Add idl, header and stubs for PerformanceEventTiming interface.
Two missing `PerformanceEntry` types that have come up in issues
are the `first-input` and the `event` entryTypes. Those are both
this.
Also, because both of those are this same interface, the static
methods from the parent class are difficult to implement because
of instance-specific details. Might either need subclasses or to
edit the parent and also everything that inherits from it :/
