rsocket - adds Single.never() (#43603)

types/rsocket-flowable/Single.d.ts

@@ -47,6 +47,7 @@ export interface IFutureSubject<T> {
 export default class Single<T> {
     static of<U>(value: U): Single<U>;
     static error(error: Error): Single<never>;
+    static never(): Single<never>;
     constructor(source: Source<T>);
     subscribe(partialSubscriber?: Partial<IFutureSubscriber<T>>): void;
     flatMap<R>(fn: (data: T) => Single<R>): Single<R>;