Introduce t_isalnum() to replace t_isalpha() || t_isdigit() tests.

Introduce t_isalnum() to replace t_isalpha() || t_isdigit() tests.

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