(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

package.json

@@ -59,6 +59,7 @@
     "@types/tmp": "0.0.33",
     "@types/uuid": "3.4.4",
     "@types/which": "2.0.1",
+    "@types/ws": "^6",
     "app-module-path": "2.2.0",
     "catw": "1.0.1",
     "chai": "4.2.0",