CLDR-15955 Add constraints for unit IDs

[macchiati]

CLDR-15955

Note that this deviates somewhat from the ticket: simple_unit is now above prefixed_unit, since the names and structure make more sense that way.

(Also, the table in the ticket got gummed up because of the presence of | characters in the text. The sheet https://docs.google.com/spreadsheets/d/1QpVPnIgJdYWZil2sr_L4P3z6O_AsT3sKZhP_SOyTVqQ/edit#gid=1228405746 has a cleaned up version.)

I considered changing the table structure to markdown, but refrained because that would make it harder to review.

• [ ] This PR completes the ticket.