Use the [FlyString] extended attribute to allow these functions to take
an Optional<FlyString> directly, allowing us to tidy up some conversions
from Optional<String>.
NewAKString is effectively the default for any new IDL interface, so
let's mark this as the default behavior. It also makes it much easier to
figure out whatever interfaces are still left to port over to new AK
String.
The intent is to use these to autogenerate prototype declarations for
Window and WorkerGlobalScope classes.
And the spec links are just nice to have :^)
