There is notably FIXME notations in this commit regarding Unicode locale
extensions. We are not parsing extensions (or private use extensions) at
all yet.
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/