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/events/tests/symbols.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var common = require('./common');
+var EventEmitter = require('../');
+var assert = require('assert');
+
+var ee = new EventEmitter();
+var foo = Symbol('foo');
+var listener = common.mustCall();
+
+ee.on(foo, listener);
+assert.strictEqual(ee.listeners(foo).length, 1);
+assert.strictEqual(ee.listeners(foo)[0], listener);
+
+ee.emit(foo);
+
+ee.removeAllListeners();
+assert.strictEqual(ee.listeners(foo).length, 0);
+
+ee.on(foo, listener);
+assert.strictEqual(ee.listeners(foo).length, 1);
+assert.strictEqual(ee.listeners(foo)[0], listener);
+
+ee.removeListener(foo, listener);
+assert.strictEqual(ee.listeners(foo).length, 0);