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/modules/esnext.set.join.js

@@ -0,0 +1,23 @@
+'use strict';
+var $ = require('../internals/export');
+var uncurryThis = require('../internals/function-uncurry-this');
+var aSet = require('../internals/a-set');
+var iterate = require('../internals/set-iterate');
+var toString = require('../internals/to-string');
+
+var arrayJoin = uncurryThis([].join);
+var push = uncurryThis([].push);
+
+// `Set.prototype.join` method
+// https://github.com/tc39/proposal-collection-methods
+$({ target: 'Set', proto: true, real: true, forced: true }, {
+  join: function join(separator) {
+    var set = aSet(this);
+    var sep = separator === undefined ? ',' : toString(separator);
+    var array = [];
+    iterate(set, function (value) {
+      push(array, value);
+    });
+    return arrayJoin(array, sep);
+  }
+});