package.json: add first-line-regex

i made this more general than the embedded shebang detection

probably want to revisit the embedded shebangs to figure out if we even *want* that

package.json

@@ -18,6 +18,7 @@
   },
   "tree-sitter": [
     {
+      "first-line-regex": "^#!.*\\bnix-shell\\b.*$",
       "injection-regex": "^(nix[-_]?shell)$",
       "scope": "source.nix_shell"
     }