Update default registry design doc to reflect LDML45 scope

[aphillips]

In the 2024-01-15 call we agree that the functions listed in this document are the comprehensive set for the LDML45 default registry. This updates our documentation to reflect that.

The list of options, operands, etc. remain open for discussion, but no other functions will be considered.

[eemeli]

I'm presuming that the "assignment" of this PR was intended as a request for reviews?

[macchiati]

FYI Something that is often forgotten is that date literal does not map to a unique start/end unless it is accompanied by a timezone.

[aphillips]

@macchiati Oh yes, we know.

I'm engaged in an update of our old friend "Working With Timezones". The editor's copy is:

https://w3c.github.io/timezone/

This is one of the areas where we did significant work at Amazon on our message formatter. The ability to float and unfloat time values and manage time zones is important in being able to get consistent and accurate results. Sections 3 and 4 of the above document capture some of this work (although you have to read between the lines to see how it plays into MF2).

[aphillips]

@macchiati

(PS> Unless I'm missing something all of this is covered by the existing options on :datetime, :date, or :time. If something is missing, please raise an issue for it.)

[aphillips]

Can I get an additional ship-it so we can merge this?