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.
Adds the initial implementation for interfaces defined in the
ResizeObserver specification. These interfaces will be used to
construct and send observation events in the upcoming changes.
