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/modules/esnext.bigint.range.js

@@ -0,0 +1,15 @@
+'use strict';
+/* eslint-disable es/no-bigint -- safe */
+var $ = require('../internals/export');
+var NumericRangeIterator = require('../internals/numeric-range-iterator');
+
+// `BigInt.range` method
+// https://github.com/tc39/proposal-Number.range
+// TODO: Remove from `core-js@4`
+if (typeof BigInt == 'function') {
+  $({ target: 'BigInt', stat: true, forced: true }, {
+    range: function range(start, end, option) {
+      return new NumericRangeIterator(start, end, option, 'bigint', BigInt(0), BigInt(1));
+    }
+  });
+}