dolthub/dolt#10113: Fix DELETE queries with NOT EXISTS uninitialized subqueries

[elianddb]

Fixes dolthub/dolt#10113
DELETE queries with NOT EXISTS subqueries failed because EXISTS expressions did not set the refsSubquery flag. This caused DELETE queries to use a simplified analyzer batch that skipped subquery initialization, leaving subqueries without execution builders.

• Refactor EXISTS expression building to reuse buildScalar() for *ast.Subquery, ensuring refsSubquery is set.
• Capture the table node for simple DELETE queries before buildWhere() wraps it.
• Separate concerns between explicit targets and implicit targets in a bool, but keep all targets in the same list to handle wrapped targets.
• Add WithTargets() method to update targets without changing the explicit/implicit flag.
• Fix offsetAssignIndexes() fast path to skip when virtual columns are present, using the full indexing path instead.

[jycor]

LGTM