New option onlyOperationTypes for typescript and flow

[standy]

This is a new config option onlyOperationTypes intended to generate only types for operations (enums, scalars, etc..., #3475 (comment) may be related)

Non-operation types are incorrect to use for the client-side. You can't get a value of the type if you don't have an operation for that. But since non-operation types are not marked, my coworkers accidentally using them all the time. That option will get rid of that

When using preResolveTypes you probably don't need all base types, so now you can skip them and significantly reduce the size of types file. For example, in dev-test/star-wars 384 lines dropped to 183 with preResolveTypes + onlyOperationTypes
dev-test/star-wars/types.ts
dev-test/star-wars/types.preResolveTypes.onlyOperationTypes.ts

I think probably onlyOperationTypes + preResolveTypes should be default for client-side types on the next major release. It makes types smaller and avoids incorrect usage.

[standy]

@dotansimha please could you take a look on this MR.
I really want this feature for all my projects. Any advice and suggestions will be greatly appreciated

[dotansimha]

@standy Sorry for the delay, I will look into it soon :)

[dotansimha]

This is awesome, thank you so much @standy .
I agree with you, in the next version we are probably change the default behaviour to that.

[havenchyk]

amazing guys, thank you so much!