Hover/autocomplete with CTEs

[reteps]

Pulling this out of #528

Hover definitions and intellisense on CTEs would be extremely helpful - this is the majority of our complex SQL code and we don't get any nice features. Additionally, most of these CTEs are basically just filtered tables with no column changes (if that type of incremental feature is easier to implement).

E.g.

WITH foo AS (SELECT * FROM table)
SELECT foo.id;

[reteps]

Any interest in doing this @psteinroe ? This would be so so helpful for us.