Add VSCode CosmosDB extension recommendation

The VSCode CosmosDB supports locally installed MongoDB servers
(and lot of other things and features).
It seems reasonable to offer that extension to project users.

Thanks!

.vscode/extensions.json

@@ -1,6 +1,7 @@
 {
     "recommendations": [
         "eg2.tslint",
+        "ms-azuretools.vscode-cosmosdb",
         "streetsidesoftware.code-spell-checker"
     ]
 }