Add '| undefined' to optional properties

[merrywhether]

Address #447, adding | undefined to all optional property types. Previously considered only updating input types (see #491 and #492), but after more consideration it seems that updating all optional types is safer and guarantees consumers don't run into issues however they choose to use the generated types. This also represents the most targeted changeset after learning a lot about the repo and TS factory types in the earlier versions.

See the snapshot to visualize the changes to the generated types. This change will be invisible to consumers who don't have exactOptionalPropertyTypes enabled, as foo?: Bar is already equivalent to foo?: Bar | undefined for them.

Also:

• turns on exactOptionalPropertyTypes in this repo (and a few associated type updates)
• 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()

[saihaj]

Thanks!!! @merrywhether

[github-actions]

🚀 PR was released in v14.3.0 🚀