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/modules/esnext.weak-map.get-or-insert-computed.js

@@ -0,0 +1,24 @@
+'use strict';
+var $ = require('../internals/export');
+var aCallable = require('../internals/a-callable');
+var aWeakMap = require('../internals/a-weak-map');
+var aWeakKey = require('../internals/a-weak-key');
+var WeakMapHelpers = require('../internals/weak-map-helpers');
+
+var get = WeakMapHelpers.get;
+var has = WeakMapHelpers.has;
+var set = WeakMapHelpers.set;
+
+// `WeakMap.prototype.getOrInsertComputed` method
+// https://github.com/tc39/proposal-upsert
+$({ target: 'WeakMap', proto: true, real: true, forced: true }, {
+  getOrInsertComputed: function getOrInsertComputed(key, callbackfn) {
+    aWeakMap(this);
+    aWeakKey(key);
+    aCallable(callbackfn);
+    if (has(this, key)) return get(this, key);
+    var value = callbackfn(key);
+    set(this, key, value);
+    return value;
+  }
+});