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.

El issue no menciona archivos, pruebas ni puntos de entrada. Empieza localizando los puntos de entrada del manejo de features del parser y las pruebas existentes del parser; después, define cómo se seleccionan los conjuntos de features compatibles y cómo se rechaza el SQL no compatible antes de que llegue a la aplicación.

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