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/es.number.is-safe-integer.js

@@ -0,0 +1,13 @@
+'use strict';
+var $ = require('../internals/export');
+var isIntegralNumber = require('../internals/is-integral-number');
+
+var abs = Math.abs;
+
+// `Number.isSafeInteger` method
+// https://tc39.es/ecma262/#sec-number.issafeinteger
+$({ target: 'Number', stat: true }, {
+  isSafeInteger: function isSafeInteger(number) {
+    return isIntegralNumber(number) && abs(number) <= 0x1FFFFFFFFFFFFF;
+  }
+});