feat(error): add known error codes constants

[aerialls]

This PR adds constants for known Postgres error codes. This allows more easily to perform the following checks without the need to look at the documentation for the associated codes.

	if err != nil {
		var pgErr *pgconn.PgError
		if errors.As(err, &pgErr) {
			if pgErr.Code == pgconn.PgErrorUniqueViolationCode {
				// ...
			}
			
		}

		return nil, err
	}

The list was extracted from the official postgres repository.

[jackc]

I think this repo will do what you want: https://github.com/jackc/pgerrcode