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/modules/initial.js

@@ -0,0 +1,8 @@
+import { slice } from './_setup.js';
+
+// Returns everything but the last entry of the array. Especially useful on
+// the arguments object. Passing **n** will return all the values in
+// the array, excluding the last N.
+export default function initial(array, n, guard) {
+  return slice.call(array, 0, Math.max(0, array.length - (n == null || guard ? 1 : n)));
+}