Adds the HTTP server response method writeProcessing for node. (#40598)

types/node/http.d.ts

@@ -157,6 +157,7 @@ declare module "http" {
         writeContinue(callback?: () => void): void;
         writeHead(statusCode: number, reasonPhrase?: string, headers?: OutgoingHttpHeaders): this;
         writeHead(statusCode: number, headers?: OutgoingHttpHeaders): this;
+        writeProcessing(): void;
     }
 
     interface InformationEvent {

types/node/test/http.ts

@@ -71,6 +71,9 @@ import * as net from 'net';
     res.writeHead(200, { 'Transfer-Encoding': 'chunked' });
     res.writeHead(200);
 
+    // writeProcessing
+    res.writeProcessing();
+
     // write string
     res.write('Part of my res.');
     // write buffer