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/underscore/cjs/countBy.js

@@ -0,0 +1,11 @@
+var _has = require('./_has.js');
+var _group = require('./_group.js');
+
+// Counts instances of an object that group by a certain criterion. Pass
+// either a string attribute to count by, or a function that returns the
+// criterion.
+var countBy = _group(function(result, value, key) {
+  if (_has(result, key)) result[key]++; else result[key] = 1;
+});
+
+module.exports = countBy;