Support more correlated subqueries and complex `EXISTS` in DML

[KKould]

Related umbrella: #130

Current state

Some DML subquery cases already work, but correlated subqueries and more complex EXISTS combinations are still incomplete.

Examples of current gaps include:

• correlated subqueries in DML filters
• more complex EXISTS shapes
• EXISTS (select * ...) against multi-column subqueries

Expected

Broaden DML support for correlated subqueries and complex EXISTS filters.

Pointers

• tests/slt/crdb/update.slt:36
• tests/slt/crdb/update.slt:84
• tests/slt/crdb/update.slt:114
• src/binder/select.rs:1700