[BUG] `x days` syntax

[sarahyurick]

Currently, something like

SELECT * FROM d_table WHERE d_date BETWEEN '2000-02-01' AND (CAST('2000-02-01' AS DATE) + 90 days

fails with a ParserError("Expected ), found: days"). Fixing this will require changes to SQLParser to allow this to work in Dask-SQL.

Note that something like

SELECT * FROM d_table WHERE d_date BETWEEN '2000-02-01' AND (CAST('2000-02-01' AS DATE) + INTERVAL '90 days'

works.