Add hello-nebula and YouTube-dislike extensions

Add two Nebula extension packages:

- hello-nebula: a minimal demo extension that shows a small badge on pages (manifest, content.js, content.css, README).
- return-youtube-dislike: a content-script port that shows estimated YouTube dislike counts using the public Return YouTube Dislike API (manifest, content.js, content.css, README). Includes GPL-3.0 LICENSE and required attribution. READMEs contain install instructions and notes.

return-youtube-dislike/manifest.json

@@ -0,0 +1,21 @@
+{
+  "id": "return-youtube-dislike",
+  "name": "Return YouTube Dislike",
+  "version": "1.0.0",
+  "description": "Shows YouTube dislike counts using the Return YouTube Dislike API. Based on https://github.com/Anarios/return-youtube-dislike",
+  "enabled": true,
+  "content_scripts": [
+    {
+      "matches": [
+        "*://*.youtube.com/*",
+        "*://youtube.com/*",
+        "*://*.youtu.be/*",
+        "*://youtu.be/*",
+        "*://*.youtube-nocookie.com/*"
+      ],
+      "js": ["content.js"],
+      "css": ["content.css"],
+      "run_at": "document_end"
+    }
+  ]
+}