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/modules/es.date.to-string.js

@@ -0,0 +1,20 @@
+'use strict';
+// TODO: Remove from `core-js@4`
+var uncurryThis = require('../internals/function-uncurry-this');
+var defineBuiltIn = require('../internals/define-built-in');
+
+var DatePrototype = Date.prototype;
+var INVALID_DATE = 'Invalid Date';
+var TO_STRING = 'toString';
+var nativeDateToString = uncurryThis(DatePrototype[TO_STRING]);
+var thisTimeValue = uncurryThis(DatePrototype.getTime);
+
+// `Date.prototype.toString` method
+// https://tc39.es/ecma262/#sec-date.prototype.tostring
+if (String(new Date(NaN)) !== INVALID_DATE) {
+  defineBuiltIn(DatePrototype, TO_STRING, function toString() {
+    var value = thisTimeValue(this);
+    // eslint-disable-next-line no-self-compare -- NaN check
+    return value === value ? nativeDateToString(this) : INVALID_DATE;
+  });
+}