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/istanbul-lib-coverage/lib/percent.js

@@ -0,0 +1,15 @@
+/*
+ Copyright 2012-2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+'use strict';
+
+module.exports = function percent(covered, total) {
+    let tmp;
+    if (total > 0) {
+        tmp = (1000 * 100 * covered) / total;
+        return Math.floor(tmp / 10) / 100;
+    } else {
+        return 100.0;
+    }
+};