fix: COMMENT ON syntax

[ragchuck]

Hi @xnuinside,

thank you for this parser!

In my tests, I couldn't get the "COMMENT ON" syntax to parse successfully. So, I tried to fix it by myself.
It seems to work now. I added correspondent tests. All tests have passed and pre-commit was executed.

This also solves #173 and potentially also #269 and #297

Please have look if you have time and let me know what you think.

Test:

def test_comment_on_columns():
    ddl = """
    CREATE TABLE users (
        id SERIAL PRIMARY KEY,
        name VARCHAR(100)
    );

    COMMENT ON COLUMN users.id IS 'Primary key for user identification';
    COMMENT ON COLUMN users.name IS 'User full name';
    """

    parse_result = DDLParser(ddl).run()
    expected = [
        {
            "columns": [
                {
                    "name": "id",
                    "type": "SERIAL",
                    "size": None,
                    "references": None,
                    "unique": False,
                    "nullable": False,
                    "default": None,
                    "check": None,
                    "comment": "Primary key for user identification",
                },
                {
                    "name": "name",
                    "type": "VARCHAR",
                    "size": 100,
                    "references": None,
                    "unique": False,
                    "nullable": True,
                    "default": None,
                    "check": None,
                    "comment": "User full name",
                },
            ],
            "primary_key": ["id"],
            "alter": {},
            "checks": [],
            "index": [],
            "schema": None,
            "partitioned_by": [],
            "table_name": "users",
            "tablespace": None,
        }
    ]
    assert expected == parse_result

Original error:

E           simple_ddl_parser.ddl_parser.DDLParserError: Unknown statement at LexToken(COMMENT,'COMMENT',1,0)

With this PR, the columns (and tables) get their "comment" attribute filled correctly through corresponding "COMMENT ON" statements.

[ragchuck]

I did some code simplification and made it that all tests are passing. I also added tests for 'comment on' syntax.