Request to Add Support for `UNION ALL` Keyword

[vxwong]

Problem:

Would it be possible to add support for UNION ALL? Since it's quite common in sql queries.

Reproduce:

here's my gold file, btw the script works well if replacing UNION ALL with UNION:

SELECT DISTINCT country FROM singer WHERE age  >  20 UNION ALL SELECT DISTINCT country FROM singer WHERE age  >  20	concert_singer

Here's the exception:

    assert toks[idx] == 'select', "'select' not found"
AssertionError: 'select' not found