Move the remaining LibJS primitive string users to UTF-16 views and
strings. Remove the primitive string UTF-8 accessors and byte-string
coercion paths so new callers cannot rely on the old storage model.
Keep primitive string storage in Utf16String and remove the UTF-8
storage path from PrimitiveString. ASCII strings still use compact
Utf16String ASCII storage, while UTF-16 becomes the only owned
representation.
Previously, the LibWeb bindings generator would output multiple per
interface files like Prototype/Constructor/Namespace/GlobalMixin
depending on the contents of that IDL file.
This complicates the build system as it means that it does not know
what files will be generated without knowledge of the contents of that
IDL file.
Instead, for each IDL file only generate a single Bindings/<IDLFile>.h
and Bindings/<IDLFile>.cpp.
I had completely managed to forget about the special case of Window
where a WindowProxy is what is received over IDL. This was caught
by the origin-from-window WPT test, but unfortunately this cannot
be imported as it relies on a web server running and other
surroundsing WPT infrastructure.
