`unnestExistsSubqueries` is a whole mess

There's a lot of correctness issues in `unnestExistsSubqueries` but we need a lot more test coverage to verify these correctness issues.
Also too many functions have confusingly similar names.

TODOs added in dolthub/go-mysql-server#3427

`unnestExistsSubqueries` also doesn't work correctly because it runs after we push outerscope filters below the SubqueryAlias (see #10490) (rule runs as part of `resolveSubqueries`). We should rearrange how our rules run but we need to be careful to make sure all the rules still run correctly.

related child issue: #10493