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/yargs/build/lib/utils/which-module.js

@@ -0,0 +1,10 @@
+export default function whichModule(exported) {
+    if (typeof require === 'undefined')
+        return null;
+    for (let i = 0, files = Object.keys(require.cache), mod; i < files.length; i++) {
+        mod = require.cache[files[i]];
+        if (mod.exports === exported)
+            return mod;
+    }
+    return null;
+}