Move helper types to internal module

[jsiebern]

I have moved the helper values / types for hover information into the module Z__INTERNAL as discussed with @jfrolich .

I also added hover support for arguments used on fields:

As well as variables that are used on field arguments:

[jsiebern]

Also just added some hover information to [%graphql. @jfrolich Let's gladly discuss if that makes sense or what other / changed information should be displayed there.

Current placeholder content in full:

---

--- GraphQL PPX Module ---

The contents of this module are automatically generated by graphql-ppx.
The following is simply an overview of the most important variables and types that you can access from this module.

module MyQuery {
  // This is the stringified representation of your query, which gets sent to the server.
  let query: string;

  // This is the main type of the result you will get back.
  // You can hover above the identifier key (e.g. query or mutation) to see the fully generated type for your module.
  type t;

  // This function turns your raw result from the server into the reason/ocaml representation of that result.
  // Depending on your graphql client library, this process should happen automatically for you.
  let parse: Raw.t => t;

  // This function will prepare your data for sending it back to the server.
  // Depending on your graphql client library, this process should happen automatically for you.
  let serialize: t => Raw.t;

  // The definition tuple is primarily used to interact with client libraries.
  // The types are equivalent to: (parse, query, serialize).
  // Your client library will use these values to provide the properly parsed / serialized data for you.
  let definition: (
    Raw.t => t,
    string,
    t => Raw.t
  );

  // This is the representation of your raw result coming from the server.
  // It should not be necessary to access the types inside for normal use cases.
  module Raw: { type t; };
}

---

[jfrolich]

Merged 🎉🎉