fix(postgres): Fix UNIQUE consuming tokens

[VaggelisD]

Fixes #3774

Currently _parse_unique() attempts to parse MySQL's UNIQUE constraint (#1708), which is parsed as:

UNIQUE KEY [<key>] (<col> [,...])

However, Postgres's UNIQUE (syntax below) does not seem to define <key> so it must not preemptively consume an id_var for it:

UNIQUE [NULLS NOT DISTINCT] [(<col> [, ...])]

For this reason, this PR:

• Turns off the [<key>] parsing in Postgres
• Adds support for NOT NULLS DISTINCT as a low-lift addition to increase DDL coverage

Docs

Postgres UNIQUE | Postgres DEFAULT | MySQL UNIQUE