This commit introduces a WEB_SET_PROTOTYPE_FOR_INTERFACE macro that
caches the interface name in a local static FlyString. This means that
we only pay for FlyString-from-literal lookup once per browser lifetime
instead of every time the interface is instantiated.
Stop worrying about tiny OOMs. Work towards #20449.
While going through these, I also changed the function signature in many
places where returning ThrowCompletionOr<T> is no longer necessary.
This ports MouseEvent, UIEvent, WheelEvent, and Event to new String.
They all had a dependency to T::create() in
WebDriverConnection::fire_an_event() and therefore had to be ported in
the same commit.
