[FEATURE]: Add more dialect-agnostic types

### Describe what you want

The addition of more dialect-agnostic base types could improve the DX of implementing and sharing query helpers — things like `jsonAgg`, `getQueryFields` (a helper similar to `getTableColumns` that also accepts `Subquery` and selects, etc.

As a quick example, the latter could benefit from `type AnySelect = AnyPgSelect | AnyMySqlSelect | AnySQLiteSelect`.