The old versions were renamed to JS_DECLARE_OLD_NATIVE_FUNCTION and
JS_DEFINE_OLD_NATIVE_FUNCTION, and will be eventually removed once all
native functions were converted to the new format.
This is the start of implementing ECMA-402 in LibJS, better known as the
ECMAScript Internationalization API.
Much like Temporal this gets its own subdirectory (Runtime/Intl/) as
well as a new C++ namespace (JS::Intl) so we don't have to prefix all
the files and classes with "Intl".
https://tc39.es/ecma402/
