Support includeDeclaration in references request.

We should support this field in the references request so the client can decide if the definition should be included in the results.

> Yeah, just pointing it out as it is the default client behaviour in those two clients.
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> I've checked the spec and it looks like there is already a `context` param that can be passed to `references` to specify if the declaration of the symbol itself should be returned.
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> _Originally posted by @ogomezba in https://github.com/elixir-lang/expert/issues/131#issuecomment-3336013557_