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/isEmpty.js

@@ -0,0 +1,18 @@
+import getLength from './_getLength.js';
+import isArray from './isArray.js';
+import isString from './isString.js';
+import isArguments from './isArguments.js';
+import keys from './keys.js';
+
+// Is a given array, string, or object empty?
+// An "empty" object has no enumerable own-properties.
+export default function isEmpty(obj) {
+  if (obj == null) return true;
+  // Skip the more expensive `toString`-based type checks if `obj` has no
+  // `.length`.
+  var length = getLength(obj);
+  if (typeof length == 'number' && (
+    isArray(obj) || isString(obj) || isArguments(obj)
+  )) return length === 0;
+  return getLength(keys(obj)) === 0;
+}