support "show" queries

[staabm]

show queries should also work with type inference and syntax error validation

examples:
'show variables like "slow_query_log"'
"SHOW TABLE STATUS where collation ='utf8_mbw4'"

    private static function get_table_columns(string $table)
    {
        $db = new DB();
        $qid = $db->query('SHOW FULL COLUMNS FROM ?, [$table]);
    }