[12.x] PostgreSQL virtual columns

[tpetry]

About two weeks ago, PostgreSQL 18 was released which finally supports virtual generated columns. This PR adds support for it - in a backwards compatible manner.

The virtualAs() column modifier was officially not support for PostgreSQL and produced SQL that was invalid:

alter table "users" add column "email_lower2" text not null generated always as (lower(email));
-- ERROR:  syntax error at or near ";"
-- LINE 1: ...il_lower2" text not null generated always as (lower(email));

With PostgreSQL 18, this syntax is now supported and defaults to a virtual generated column. However, the current implementation was changed to explicitly add the virtual keyword to the DDL query - just to be sure in case defaults may change in the future.

Additionally, the tests schema builder tests have been changed to now also test virtual columns which had been excluded in the past - because PG didn't support it.

[taylorotwell]

Thanks!