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/SyntaxError.js

@@ -0,0 +1,14 @@
+var createCustomError = require('../utils/createCustomError');
+
+module.exports = function SyntaxError(message, input, offset) {
+    var error = createCustomError('SyntaxError', message);
+
+    error.input = input;
+    error.offset = offset;
+    error.rawMessage = message;
+    error.message = error.rawMessage + '\n' +
+        '  ' + error.input + '\n' +
+        '--' + new Array((error.offset || error.input.length) + 1).join('-') + '^';
+
+    return error;
+};