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.