meteor: add methods to Mongo.ObjectID

These method are accessible from meteor Mongo.ObjectID but missing.

Per https://docs.meteor.com/api/collections.html#Mongo-ObjectID :
"Mongo.ObjectID follows the same API as the Node MongoDB driver ObjectID class"

When I check the Node.js MongoDB Driver API, only these two methods
are interesting. (getTimestamp()  doesn't make sense in Meteor).

I've a test that call both methods successfully.

types/meteor/mongo.d.ts

@@ -214,7 +214,10 @@ declare module "meteor/mongo" {
         interface ObjectIDStatic {
             new (hexString?: string): ObjectID;
         }
-        interface ObjectID { }
+        interface ObjectID {
+            toHexString(): string;
+            equals(otherID: ObjectID): boolean;
+         }
 
         function setConnectionOptions(options: any): void;
     }