Nested alias

[waitingkuo]

origin nestedjoin doesn't have alias. this pr add alias for nestedjoin

[waitingkuo]

help wanted.

this pr break this test

    let res = snowflake_and_generic().parse_sql_statements("SELECT * FROM (a NATURAL JOIN b) c");
    assert_eq!(
        ParserError::ParserError("Expected end of statement, found: c".to_string()),
        res.unwrap_err()
    );

Could anyone tell me how to deal with this?

thanks~

[waitingkuo]

i made another draft pull request