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/inspect-source.js

@@ -0,0 +1,15 @@
+'use strict';
+var uncurryThis = require('../internals/function-uncurry-this');
+var isCallable = require('../internals/is-callable');
+var store = require('../internals/shared-store');
+
+var functionToString = uncurryThis(Function.toString);
+
+// this helper broken in `core-js@3.4.1-3.4.4`, so we can't use `shared` helper
+if (!isCallable(store.inspectSource)) {
+  store.inspectSource = function (it) {
+    return functionToString(it);
+  };
+}
+
+module.exports = store.inspectSource;