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-set-like.js

@@ -0,0 +1,20 @@
+'use strict';
+var getBuiltIn = require('../internals/get-built-in');
+var isCallable = require('../internals/is-callable');
+var isIterable = require('../internals/is-iterable');
+var isObject = require('../internals/is-object');
+
+var Set = getBuiltIn('Set');
+
+var isSetLike = function (it) {
+  return isObject(it)
+    && typeof it.size == 'number'
+    && isCallable(it.has)
+    && isCallable(it.keys);
+};
+
+// fallback old -> new set methods proposal arguments
+module.exports = function (it) {
+  if (isSetLike(it)) return it;
+  return isIterable(it) ? new Set(it) : it;
+};