Fix duplicate p5.js import check and preserve window.p5 constructor

Author: VANSH3104

src/core/main.js

@@ -125,12 +125,12 @@ class p5 {
     // If the user has created a global setup or draw function,
     // assume "global" mode and make everything global (i.e. on the window)
     if (!sketch) {
-      if (window.p5 instanceof p5) {
+      if (typeof window.p5 !== 'undefined') {
         console.warn('p5.js appears to have been imported multiple times. This could cause errors.');
         return;
       }
       this._isGlobal = true;
-      window.p5 = this;
+      window._p5Instance = this;
       if (window.hitCriticalError) {
         return;
       }