alter table support multiple columns

[carlvine500]

Is your feature request related to a problem? Please describe.
we always add/modify/drop multiple columns . but simple-ddl-parse output alter is empty:

def test_alter_table_add_two_column():
    ddl = """
                      CREATE TABLE material_attachments (
                        material_id int NOT NULL,
                        attachment_id int NOT NULL
                      );
                      ALTER TABLE material_attachments ADD column_name1 datatype,ADD column_name2 datatype;
                  """
    result = DDLParser(ddl).run(group_by_type=True)
    print(json.dumps(result, indent=4))

the wrong output as following (output alter is {}):

test_ddl_parser.py::test_alter_table_add_two_column PASSED               [100%]{
    "tables": [
        {
            "table_name": "material_attachments",
            "schema": null,
            "primary_key": [],
            "columns": [
                {
                    "name": "material_id",
                    "type": "int",
                    "size": null,
                    "references": null,
                    "unique": false,
                    "nullable": false,
                    "default": null,
                    "check": null
                },
                {
                    "name": "attachment_id",
                    "type": "int",
                    "size": null,
                    "references": null,
                    "unique": false,
                    "nullable": false,
                    "default": null,
                    "check": null
                }
            ],
            "alter": {},
            "checks": [],
            "index": [],
            "partitioned_by": [],
            "tablespace": null
        }
    ],
    "types": [],
    "sequences": [],
    "domains": [],
    "schemas": [],
    "ddl_properties": []
}

Describe the solution you'd like
output alter is not empty

Describe alternatives you've considered

Additional context
the same problem test case(output alter is {}):

def test_alter_table_drop():
    ddl = """
                      CREATE TABLE material_attachments (
                        material_id int NOT NULL,
                        attachment_id int NOT NULL
                      );
                      ALTER TABLE material_attachments DROP COLUMN attachment_id;
                  """
    result = DDLParser(ddl).run(group_by_type=True)
    print(json.dumps(result, indent=4))

def test_alter_table_modify_2_column():
    ddl = """
                      CREATE TABLE material_attachments (
                        material_id int NOT NULL,
                        attachment_id int NOT NULL
                      );
                      ALTER TABLE material_attachments MODIFY COLUMN material_id integer,MODIFY COLUMN attachment_id integer;
                  """
    result = DDLParser(ddl).run(group_by_type=True)
    print(json.dumps(result, indent=4))

and add coumn not supported(output alter is {}):

def test_alter_table_add_column():
    ddl = """
                      CREATE TABLE material_attachments (
                        material_id int NOT NULL,
                        attachment_id int NOT NULL
                      );
                      ALTER TABLE material_attachments ADD COLUMN column_name datatype;
                  """
    result = DDLParser(ddl).run(group_by_type=True)
    print(json.dumps(result, indent=4))