Update with-mongodb to be TypeScript-friendly

I slightly modified the approach so TypeScript can correctly infer types without actually having to type anything but the global:

**index.d.ts**
```ts
import { Db, MongoClient } from "mongodb";

declare global {
  namespace NodeJS {
    interface Global {
      mongoCache: {
        conn: {
          client: MongoClient | null;
          db: Db | null;
        }
        promise: Promise<MongoClient> | null;
      };
    }
  }
}
```

examples/with-mongodb/util/mongodb.js

@@ -20,26 +20,23 @@ if (!MONGODB_DB) {
  * during API Route usage.
  */
 let cached = global.mongo
-if (!cached) cached = global.mongo = {}
+if (!cached) cached = global.mongo = { conn: null, promise: null }
 
 export async function connectToDatabase() {
   if (cached.conn) return cached.conn
   if (!cached.promise) {
-    const conn = {}
     const opts = {
       useNewUrlParser: true,
       useUnifiedTopology: true,
     }
     cached.promise = MongoClient.connect(MONGODB_URI, opts)
       .then((client) => {
-        conn.client = client
-        return client.db(MONGODB_DB)
-      })
-      .then((db) => {
-        conn.db = db
-        cached.conn = conn
+        return {
+          client,
+          db: client.db(MONGODB_DB),
+        }
       })
   }
-  await cached.promise
+  cached.conn = await cached.promise
   return cached.conn
 }