Enable SQL features selectively

Hello,
given the breadth of the parser, it would be nice if it would be possible to selectively enable features on an opt-in basis.

Let's imagine an application where only a subset of SQL is supported. In this case, the application would need to check for every possible unsupported feature manually (e.g. if I don't support windowing in queries, I would need to check if the `named_window` field in the `Select` gets populated and abort).

It would be interesting, instead, to decide beforehand which feature set is to be supported, so that only "valid" SQL gets parsed and arrives to the application.

Thanks.

The issue names no files, tests, or entry points. Start by locating the parser's feature-handling entry points and existing parser tests, then define how supported feature sets are selected and how unsupported SQL is rejected before reaching the application.

Written by the indexing model from the issue text.