(core) Converting server-side Comm.js to typescript

Summary:
- Add app/common/CommTypes.ts to define types shared by client and server.
- Include @types/ws npm package

Test Plan: Intended to have no changes in behavior

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3467

app/server/lib/ITestingHooks.ts

@@ -8,7 +8,7 @@ export interface ITestingHooks {
   disconnectClients(): Promise<void>;
   commShutdown(): Promise<void>;
   commRestart(): Promise<void>;
-  commSetClientPersistence(ttlMs: number): Promise<void>;
+  commSetClientPersistence(ttlMs: number): Promise<number>;
   closeDocs(): Promise<void>;
   setDocWorkerActivation(workerId: string, active: 'active'|'inactive'|'crash'): Promise<void>;
   flushAuthorizerCache(): Promise<void>;