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/is-core-module/.eslintrc

@@ -0,0 +1,18 @@
+{
+    "extends": "@ljharb",
+    "root": true,
+    "rules": {
+		"func-style": 1,
+    },
+    "overrides": [
+        {
+            "files": "test/**",
+            "rules": {
+                "global-require": 0,
+				"max-depth": 0,
+                "max-lines-per-function": 0,
+				"no-negated-condition": 0,
+            },
+        },
+    ],
+}