Intl.DisplayNames was the first Intl object implemented, and at that
point all AOs were just put into the main Intl AO header. But AOs that
belong to specific objects belong in that object's header. So this moves
CanonicalCodeForDisplayNames to the Intl.DisplayNames header.
Currently, all callers of ResolveLocale invoke the operation with an
empty [[RelevantExtensionKeys]] slot, so the block of the method that
deals with those keys was unimplemented. This implements that block now
to prepare for Intl.NumberFormat which has a [[RelevantExtensionKeys]].
Note that the find_key_in_value() method is a simple VERIFY_NOT_REACHED
in just this commit until the Intl.NumberFormat's keys are handled in
its implementation.
Note that only option type=region is really implemented. Other types
will resort to the fallback option. This prototype method will be able
to implement other type options once LibUnicode supports more.
There is notably FIXME notations in this commit regarding Unicode locale
extensions. We are not parsing extensions (or private use extensions) at
all yet.
