More extensions for built-in postgres extensions, e.g. unaccent

[aljazerzen]

unaccent is a Postgres "trusted" extension, which EdgeDB should expose as an extension too.

https://www.postgresql.org/docs/current/unaccent.html

Example usage:

V2f203bc190_main=# create extension unaccent;
CREATE EXTENSION
V2f203bc190_main=# select unaccent('À');
 unaccent
----------
 A