Add license file change check.
(Since the license file changed this time,
and it's something that one should check if it happens.)

Some clean-up.
Documents that the BG9c rule from Unicode 15.10 is not supported.
(That would require more bits of information per character,
information that the current table builder does not have,
and a correspondingly larger state machine.)

Co-authored-by: Devon Carew <[email protected]>

The `Characters` class is an empty interface, but adding `interface` to it is potentially breaking, so not doing it as part of this PR.