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/index-default.js

@@ -0,0 +1,11 @@
+var mixin = require('./mixin.js');
+var index = require('./index.js');
+
+// Default Export
+
+// Add all of the Underscore functions to the wrapper object.
+var _ = mixin(index);
+// Legacy Node.js API.
+_._ = _;
+
+module.exports = _;