package.json: implement an always-failing content-regex

package.json

@@ -19,6 +19,7 @@
   "tree-sitter": [
     {
       "first-line-regex": "^#!.*\\bnix-shell\\b.*$",
+      "content-regex": "^\b$nix-shell wraps other languages so intentionally fail against (and yield to) those other language matches",
       "injection-regex": "^(nix[-_]?shell)$",
       "scope": "source.nix_shell"
     }