ts_locale.c omitted support for "isalnum" tests, perhaps on the
grounds that there were initially no use-cases for that. However,
both ltree and pg_trgm need such tests, and we do also have one
use-case now in the core backend. The workaround of testing
isalpha and isdigit separately seems quite inefficient, especially
when dealing with multibyte characters; so let's fill in the
missing support.
Discussion: https://postgr.es/m/2548310.1664999615@sss.pgh.pa.us
ca71131eeb