WIP: Cookbook example showing custom calendar proof of concept (without custom calendar object)

[ptomato]

This is an example of how to approach implementing a custom calendar, after Temporal.Calendar objects are removed.

It's an implementation of a Nepali calendar (described by @kabaros in #2854 (comment)) based on the implementation linked in that comment, as well as the implementation that was based on.

I used inheritance in this example because in this case that's slightly less code to write, but inheritance also makes some operations legal that maybe shouldn't be. (Like, Temporal.PlainDateTime.from(nepaliPlainDate)) So it might be better to switch to composition.

I'd also like to write a similar example about Hijri dates with manual adjustment (described in #2854 (comment)) but it was easier to do the proof of concept with Nepali calendar first, since there was already an implementation to copy from. I'll be in contact with @khawarizmus about this.

Draft; needs some cleanup, and a cookbook page.

[kabaros]

thanks @ptomato - this is very helpful (and re-assuring).