Additional unicode primitive functions.

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

src/common/meson.build

@@ -30,6 +30,7 @@ common_sources = files(
   'scram-common.c',
   'string.c',
   'stringinfo.c',
+  'unicode_category.c',
   'unicode_norm.c',
   'username.c',
   'wait_error.c',