Add '| undefined' to optional input properties (option 2)

[merrywhether]

Address #447 with changes focused in TypeScriptTypeTransformers.ts (see #491 for alternate implementation, depending on maintainers' preference of approach). This adds | undefined only to input types, as those were the only sites our Relay/TS codebase complained about the generated types.

Changing this implementation to add | undefined to all optional types would be probably hybrid with #491 and would yield changes like this:

      // FriendFragment.graphql
      import { FragmentRefs } from "relay-runtime";
      export type FriendFragment = {
    -     readonly name?: string | null;
    -     readonly lastName?: string | null;
    +     readonly name?: string | null | undefined;
    +     readonly lastName?: string | null | undefined;
          readonly feedback?: {
              readonly id: string;
              readonly name: string | null;
    -     } | null;
    +     } | null | undefined;
          readonly " $refType": "FriendFragment";
      };
      export type FriendFragment$data = FriendFragment;
      export type FriendFragment$key = {
    -     readonly " $data"?: FriendFragment$data;
    +     readonly " $data"?: FriendFragment$data | undefined;
          readonly " $fragmentRefs": FragmentRefs<"FriendFragment">;
      };

I think the only time this would come up would be when trying to create test mocks? So might be worth doing as well and happy to augment, but wanted to start with the minimal changeset and work from there, especially as this version requires the addition of only a single extra conditional branch.

Also:

• turns on exactOptionalPropertyTypes in this repo
• narrows type of State.generatedInputObjectTypes (found while working in generateInputObjectTypes in Add '| undefined' to optional input properties (option 1) #491)
• removes 2 duplicate fixtures that were named mutaion instead of mutation
• replaces ts.factory.createLiteralTypeNode(ts.factory.createToken(ts.SyntaxKind.NullKeyword)) with ts.factory.createNull()

[merrywhether]

This could also be something like isPropertySignature and then require an update at the two other call sites while reverting the one modified below.

[merrywhether]

Replaced by #494