Closes #4510

Hey! As discussed in #4510, I noticed that calling DateTimeFormat.formatDateParts() creates new instances of NumberFormat on every call. As creating NumberFormat can be quite heavy due to the bestfit mode of LocaleMatcher (see #4276 for more details), it's best to cache NumberFormat instances.

To keep performance of the DateTimeFormat constructor as-is, I lazily initialize the three (sometimes just two) NumberFormat instances during the first call to formatDateParts and store them in the internalSlots of the DateTimeFormat.

I'm not 100% sure if internalSlots is the right place to store these internal instances, if not, let me know where I could store them instead.

Thanks!