fix ns.prompt typechecking

Introduced some new type assertion functions, initial step in larger type safety rework.

src/Netscript/NetscriptHelpers.ts

@@ -65,6 +65,9 @@ export const helpers = {
   failOnHacknetServer,
 };
 
+
+/** Will probably remove the below function in favor of a different approach to object type assertion.
+ *  This method cannot be used to handle optional properties. */ 
 export function assertObjectType<T extends object>(
   ctx: NetscriptContext,
   name: string,