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/proposals/set-methods.js

@@ -0,0 +1,17 @@
+'use strict';
+// https://github.com/tc39/proposal-set-methods
+require('../modules/esnext.set.difference.v2');
+require('../modules/esnext.set.intersection.v2');
+require('../modules/esnext.set.is-disjoint-from.v2');
+require('../modules/esnext.set.is-subset-of.v2');
+require('../modules/esnext.set.is-superset-of.v2');
+require('../modules/esnext.set.union.v2');
+require('../modules/esnext.set.symmetric-difference.v2');
+// TODO: Obsolete versions, remove from `core-js@4`
+require('../modules/esnext.set.difference');
+require('../modules/esnext.set.intersection');
+require('../modules/esnext.set.is-disjoint-from');
+require('../modules/esnext.set.is-subset-of');
+require('../modules/esnext.set.is-superset-of');
+require('../modules/esnext.set.union');
+require('../modules/esnext.set.symmetric-difference');