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/css-tree/lib/syntax/function/element.js

@@ -0,0 +1,13 @@
+// https://drafts.csswg.org/css-images-4/#element-notation
+// https://developer.mozilla.org/en-US/docs/Web/CSS/element
+module.exports = function() {
+    this.scanner.skipSC();
+
+    var children = this.createSingleNodeList(
+        this.IdSelector()
+    );
+
+    this.scanner.skipSC();
+
+    return children;
+};