Yaroslav Kukytsyak

- Replace Node 15 in tests with Node 16. - Run integration tests also on TypeScript 4.2 and 4.3.

- Improve formatting of queries in tests. - Improve test descriptions. - Compare uninitialized variables against `undefined`. Comparing against `null` will pass whether the variables were assigned the request/response object...

**Changes:** - Fix typos and improve some comments. - Add `@deprecated` tag to `formatError` comment so that some editors can show that this option is deprecated by, e.g., a strikethrough.

`MaybePromise` can be misinterpreted as `Maybe`. Naming this type as `PromiseOrValue` makes it clear what this type represents and prevents the aforementioned possible misinterpretation. `PromiseOrValue` is also defined in `graphql-js`...