handle embedded script payload

corpus/payload.txt

@@ -0,0 +1,25 @@
+==================
+nix-shell with a bash payload
+==================
+
+#!/usr/bin/env nix-shell
+#!nix-shell -i bash
+echo "hello, world!"
+---
+
+(source_file
+  (first_line
+    (shebang_open)
+    (opt_ws)
+    (env)
+    (ws)
+    (nix_shell))
+  (next_line
+    (newline)
+    (annotation_line
+      (shebang_open)
+      (opt_ws)
+      (nix_shell)
+      (nix_shell_args)))
+  (next_line
+    (newline)))

corpus/shebang_with_sub_interpreter.txt

@@ -13,7 +13,7 @@ Minimal nix-shell invocation
     (env)
     (ws)
     (nix_shell))
-  (non_first_line
+  (next_line
     (newline)
     (annotation_line
       (shebang_open)