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/es-abstract/2018/IsConcatSpreadable.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $isConcatSpreadable = GetIntrinsic('%Symbol.isConcatSpreadable%', true);
+
+var Get = require('./Get');
+var IsArray = require('./IsArray');
+var ToBoolean = require('./ToBoolean');
+
+var isObject = require('es-object-atoms/isObject');
+
+// https://262.ecma-international.org/6.0/#sec-isconcatspreadable
+
+module.exports = function IsConcatSpreadable(O) {
+	if (!isObject(O)) {
+		return false;
+	}
+	if ($isConcatSpreadable) {
+		var spreadable = Get(O, $isConcatSpreadable);
+		if (typeof spreadable !== 'undefined') {
+			return ToBoolean(spreadable);
+		}
+	}
+	return IsArray(O);
+};