chore: Clarify usage of window fields in UIPrompt.js

/puter/src/UI/UIPrompt.js
  26:12  error  'isString' is not defined  no-undef

src/UI/UIPrompt.js

@@ -23,7 +23,7 @@ function UIPrompt(options){
     // set sensible defaults
     if(arguments.length > 0){
         // if first argument is a string, then assume it is the message
-        if(isString(arguments[0])){
+        if(window.isString(arguments[0])){
             options = {};
             options.message = arguments[0];
         }