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/range.js

@@ -0,0 +1,23 @@
+// Generate an integer Array containing an arithmetic progression. A port of
+// the native Python `range()` function. See
+// [the Python documentation](https://docs.python.org/library/functions.html#range).
+function range(start, stop, step) {
+  if (stop == null) {
+    stop = start || 0;
+    start = 0;
+  }
+  if (!step) {
+    step = stop < start ? -1 : 1;
+  }
+
+  var length = Math.max(Math.ceil((stop - start) / step), 0);
+  var range = Array(length);
+
+  for (var idx = 0; idx < length; idx++, start += step) {
+    range[idx] = start;
+  }
+
+  return range;
+}
+
+module.exports = range;