Initial commit: Developer Tools MVP with visual editor

- Complete React app with 7 developer tools
- JSON Tool with visual structured editor
- Serialize Tool with visual structured editor
- URL, Base64, CSV/JSON, Beautifier, Diff tools
- Responsive navigation with dropdown menu
- Dark/light mode toggle
- Mobile-responsive design with sticky header
- All tools working with copy/paste functionality

node_modules/core-js-pure/internals/map-upsert.js

@@ -0,0 +1,31 @@
+'use strict';
+var call = require('../internals/function-call');
+var aCallable = require('../internals/a-callable');
+var isCallable = require('../internals/is-callable');
+var anObject = require('../internals/an-object');
+
+var $TypeError = TypeError;
+
+// `Map.prototype.upsert` method
+// https://github.com/tc39/proposal-upsert
+module.exports = function upsert(key, updateFn /* , insertFn */) {
+  var map = anObject(this);
+  var get = aCallable(map.get);
+  var has = aCallable(map.has);
+  var set = aCallable(map.set);
+  var insertFn = arguments.length > 2 ? arguments[2] : undefined;
+  var value;
+  if (!isCallable(updateFn) && !isCallable(insertFn)) {
+    throw new $TypeError('At least one callback required');
+  }
+  if (call(has, map, key)) {
+    value = call(get, map, key);
+    if (isCallable(updateFn)) {
+      value = updateFn(value);
+      call(set, map, key, value);
+    }
+  } else if (isCallable(insertFn)) {
+    value = insertFn();
+    call(set, map, key, value);
+  } return value;
+};