Move the overload-resolution metadata types into LibWeb::WebIDL and
update the Python generator and overload resolver to use them.
LibIDL no longer has any users after the C++ bindings generator removal,
so remove the library and unlink it from LibWeb.
This has quite a lot of fall out. But the majority of it is just type or
UDL substitution, where the changes just fall through to other function
calls.
By changing property key storage to UTF-16, the main affected areas are:
* NativeFunction names must now be UTF-16
* Bytecode identifiers must now be UTF-16
* Module/binding names must now be UTF-16
