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/core-js/internals/to-absolute-index.js

@@ -0,0 +1,13 @@
+'use strict';
+var toIntegerOrInfinity = require('../internals/to-integer-or-infinity');
+
+var max = Math.max;
+var min = Math.min;
+
+// Helper for a popular repeating case of the spec:
+// Let integer be ? ToInteger(index).
+// If integer < 0, let result be max((length + integer), 0); else let result be min(integer, length).
+module.exports = function (index, length) {
+  var integer = toIntegerOrInfinity(index);
+  return integer < 0 ? max(integer + length, 0) : min(integer, length);
+};