Incompatibility: ALTER COLUMN DROP DEFAULT

[apstndb]

This incompatibility is detected by #90.

Spec

https://cloud.google.com/spanner/docs/reference/standard-sql/data-definition-language#alter_table

ALTER TABLE table_name
    action

where action is:
...
    ALTER [ COLUMN ] column_name
      {
        data_type  [ NOT NULL ] [ DEFAULT ( expression ) ]
        | SET OPTIONS ( options_def )
        | SET DEFAULT ( expression )
        | DROP DEFAULT
      }
...

Reproducing input

ALTER TABLE T ALTER COLUMN expr DROP DEFAULT;"

Current behavior

syntax error:testdata/column_default_values.test:6:33: expect ident: BOOL, INT64, FLOAT32, FLOAT64, DATE, TIMESTAMP, NUMERIC, JSON, STRING, BYTES, but: DROP

  6:  ALTER TABLE T ALTER COLUMN expr DROP DEFAULT;
                                      ^~~~