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
Factor out canvas parsing algorihtm for CanvasRenderingContext2DSettings
from JS::Value. This was only used in one place but needs to be usable
from other places too in the future.