Fix VariableFn<V> type returns Object instead of V

[bbugh]

The VariableFn<V> type incorrectly returns any Object type, rather than the type specified by V. This is a small change but has a high impact on type correctness.

This resulted in an easily preventable typing bug in our app. Here is a simplified example:

apollo: {
  project (): VueApolloQueryOptions<ProjectQueryVariables, ProjectQuery> {
    return {
      variables: { projectId: 5 }, // ❌ should be just id, but the type isn't checked 😢
      query: PROJECT_QUERY,
      fetchPolicy: 'network-only'
    }
  }
}

This means that the variables key of a few important things don't actually check V variables type:

• ApolloVueSubscribeToMoreOptions<V>
• VueApolloQueryOptions<V, R>
• VueApolloMutationOptions<V, R>
• VueApolloSubscriptionOptions<V, R>

[ciscorn]

V means a vue instance not variables.

This merge has broken many things as the CI results shows.