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/eslint-plugin-react/lib/rules/no-will-update-set-state.js

@@ -0,0 +1,15 @@
+/**
+ * @fileoverview Prevent usage of setState in componentWillUpdate
+ * @author Yannick Croissant
+ */
+
+'use strict';
+
+const makeNoMethodSetStateRule = require('../util/makeNoMethodSetStateRule');
+const testReactVersion = require('../util/version').testReactVersion;
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = makeNoMethodSetStateRule(
+  'componentWillUpdate',
+  (context) => testReactVersion(context, '>= 16.3.0')
+);