Generate (correct) TypeScript and Flow declaration files

This change adds Flow declaration types so that we can check the types
of the generated code.

Notably:

- Adds a dependency to flowgen and calls it during compilation, which
  takes the `.d.ts` and creates a `.flow.js` based on it.
- Passes in the `--force-number` flag to pbjs, which causes it to not
  emit numbers as possibly being `Long`, since this confuses Flow. This
  only affects JSDoc declarations, not the generated code.
- Passes in the `--force-message` flag to pbjs. Otherwise, this emits
  invalid types: it tries to create code like this:

  ```TypeScript
  interface IMessage {
    field?: T|null;
  }

  class Message implements IMessage {
    field: T;  // notice the lack of optionality / nullness.
    ...
  }
  ```

  This violates the Liskov Substitution Principle, since `Message`
  cannot be passed to any place that accepts an `IMessage`, so
  TypeScript (correctly) rejects this as bogus. With this flag, the
  number of places that will accept an `IMessage` instead of a `Message`
  are limited to constructors, where we do want to have partial messages
  being passed in.

  This only affects JSDoc declarations, not the generated code.
- Removes all `@implements` annotations from the JSDoc, which takes care
  of the very last `IMessage`/`Message` confusion. This, together with
  the above element take care of
  protobufjs/protobuf.js#837