Make codegenScript handle invocations via async <<__Entrypoint>>s

If no script is provided, we were inferring it from the last entry in
the backtrace.

If you are using async `<<__Entrypoint>>`, the first entry in the call
stack is in systemlib, in enterasyncentrypoint.
- systemlib doesn't have file names in backtraces as they're considered
  builtin. This caused the crash
- even if it did, it would be the wrong file name

So, choose the last thing with a filename.

Author: fredemmott

src/CodegenFactoryTrait.php

@@ -202,10 +202,11 @@ final public function codegenGeneratedFromScript(
     ?string $script = null,
   ): CodegenGeneratedFrom {
     if ($script === null) {
-      $trace = \debug_backtrace();
-      $last = C\lastx($trace);
+      $last = \debug_backtrace()
+        |> Vec\filter($$, $frame ==> C\contains_key($frame, 'file'))
+        |> C\last($$);
       invariant(
-        $last !== false,
+        $last !== null,
         "Couldn't get the strack trace.  Please pass the script name to ".
         "codegenGeneratedFromScript",
       );