Sync tstypes/errors with flow

[JacksonKearl]

One remaining diff is that in TS, we define formatError as

export interface GraphQLFormattedError<
  TExtensions extends Record<string, any> = Record<string, any>
> {
  readonly message: string;
  readonly locations?: ReadonlyArray<SourceLocation>;
  readonly path?: ReadonlyArray<string | number>;
  readonly extensions?: TExtensions;
}

whereas in flow, we define it like

export type GraphQLFormattedError = {|
  +message: string,
  +locations: $ReadOnlyArray<SourceLocation> | void,
  +path: $ReadOnlyArray<string | number> | void,
  +extensions?: { [key: string]: mixed, ... },
|};

(comments stripped)

Don't have much flow expereince, but playing in the playground, its saying that adding a type parameter is a breaking change (must use GraphQLFormattedError<>, even though the type has a defualt)

[IvanGoncharov]

@JacksonKearl Merged 🎉
Thanks a lot for separating .d.ts comments into a separate commit 👍