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/definition-syntax/walk.js

@@ -0,0 +1,52 @@
+var noop = function() {};
+
+function ensureFunction(value) {
+    return typeof value === 'function' ? value : noop;
+}
+
+module.exports = function(node, options, context) {
+    function walk(node) {
+        enter.call(context, node);
+
+        switch (node.type) {
+            case 'Group':
+                node.terms.forEach(walk);
+                break;
+
+            case 'Multiplier':
+                walk(node.term);
+                break;
+
+            case 'Type':
+            case 'Property':
+            case 'Keyword':
+            case 'AtKeyword':
+            case 'Function':
+            case 'String':
+            case 'Token':
+            case 'Comma':
+                break;
+
+            default:
+                throw new Error('Unknown type: ' + node.type);
+        }
+
+        leave.call(context, node);
+    }
+
+    var enter = noop;
+    var leave = noop;
+
+    if (typeof options === 'function') {
+        enter = options;
+    } else if (options) {
+        enter = ensureFunction(options.enter);
+        leave = ensureFunction(options.leave);
+    }
+
+    if (enter === noop && leave === noop) {
+        throw new Error('Neither `enter` nor `leave` walker handler is set or both aren\'t a function');
+    }
+
+    walk(node, context);
+};