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.
This does not implement any of the IDL methods, but GitHub requires the
interface exists to upload files via an <input type="file"> element.
Their JS handles uploads via this element and via drag-and-drop in one
function, and check if the uploaded file is `instanceof DataTransfer` to
decide how to handle it.