Postgres: Accept generated columns with no mode keyword or VIRTUAL

[ting-hong-shieh]

Fixes #2407.

PostgreSqlDialect required STORED after GENERATED ALWAYS AS (expr), so it rejected both the omitted mode and an explicit VIRTUAL:

CREATE TABLE users (..., name character varying(255)
    GENERATED ALWAYS AS (((first_name || ' '::text) || last_name)) NOT NULL);
-- Expected: STORED, found: NOT at Line: 6, Column: 9

CREATE TABLE users (a text, name text GENERATED ALWAYS AS (a) VIRTUAL);
-- Expected: STORED, found: VIRTUAL at Line: 1, Column: 63

PostgreSQL 18 documents the grammar as GENERATED ALWAYS AS ( generation_expr ) [ STORED | VIRTUAL ], with VIRTUAL as the default when the keyword is omitted.

This removes the PostgreSqlDialect special case so the dialect falls back to the shared handling, which already parses both forms and round-trips them unchanged. STORED is unaffected.