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-pure/internals/to-positive-integer.js

@@ -0,0 +1,10 @@
+'use strict';
+var toIntegerOrInfinity = require('../internals/to-integer-or-infinity');
+
+var $RangeError = RangeError;
+
+module.exports = function (it) {
+  var result = toIntegerOrInfinity(it);
+  if (result < 0) throw new $RangeError("The argument can't be less than 0");
+  return result;
+};