Introduce unicode_version(), icu_unicode_version(), and
unicode_assigned().
The latter requires introducing a new lookup table for the Unicode
General Category, which is generated along with the other Unicode
lookup tables.
Discussion: https://postgr.es/m/CA+TgmoYzYR-yhU6k1XFCADeyj=Oyz2PkVsa3iKv+keM8wp-F_A@mail.gmail.com
Reviewed-by: Peter Eisentraut