stubgen: Add flagfile support (#18061)

## Description

This is already supported by main `mypy` binary and `dmypy` but not by
`stubgen`.

## Context
I maintain https://github.com/KapJI/homeassistant-stubs which
automatically generates typing stubs. Currently list of arguments there
is 50 KB long and this can be longer that supported by OS (depending on
configuration). It'd help to write these args to flagfile.

mypy/stubgen.py

@@ -1793,7 +1793,9 @@ def generate_stubs(options: Options) -> None:
 
 
 def parse_options(args: list[str]) -> Options:
-    parser = argparse.ArgumentParser(prog="stubgen", usage=HEADER, description=DESCRIPTION)
+    parser = argparse.ArgumentParser(
+        prog="stubgen", usage=HEADER, description=DESCRIPTION, fromfile_prefix_chars="@"
+    )
 
     parser.add_argument(
         "--ignore-errors",