[V1] Fix handling of subqueries and VALUES modeling

[RCHowell]

Description

Previous versions of PartiQL did not handle modeling of sub-queries beyond treating them like collections, but there are additional SQL constructs we need to account for. The V1 plans contain the expanded modeling, but the AST and planner need to support these. You'll find several TODOs commented into the codebase, this issue is to track completion of end-to-end sub-query handling per SQL spec.

Related recent PR #1638

need to take a closer look at the VALUES modeling. I'm not sure yet if it should be an Expr or something different.