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/underscore/cjs/union.js

@@ -0,0 +1,11 @@
+var _flatten = require('./_flatten.js');
+var restArguments = require('./restArguments.js');
+var uniq = require('./uniq.js');
+
+// Produce an array that contains the union: each distinct element from all of
+// the passed-in arrays.
+var union = restArguments(function(arrays) {
+  return uniq(_flatten(arrays, true, true));
+});
+
+module.exports = union;