[ElementInternalsType] needs declarative interface

[brianleroux]

Pleased to see work returning to having the ability to extend builtins but was disappointed no consideration for a declarative interface. It would be unfortunate if the only way to access this capability was via clientside JavaScript. Many use cases do not require client js and it would be good to support progressive enhancement by default. If this isn't considered now it could be very awkward to bolt on later.