chore: add remark and plugin keywords in _tools/remark/plugins/remark-stdlib-urls-github

The package is a remark plugin yet lacked the `remark` and `plugin`
keywords carried by 11 of 14 sibling packages in
`@stdlib/_tools/remark/plugins`. Adding both restores the namespace
keyword convention.

Author: claude

lib/node_modules/@stdlib/_tools/remark/plugins/remark-stdlib-urls-github/package.json

@@ -41,6 +41,8 @@
     "stdlib",
     "repository",
     "repo",
-    "github"
+    "github",
+    "remark",
+    "plugin"
   ]
 }