patch 9.1.1342: Shebang filetype detection can be improved

patch 9.1.1342: Shebang filetype detection can be improved

Problem:  Shebang filetype detection can be improved
Solution: Improve detection logic (Eisuke Kawashima)

Vim does not correctly detect filetype from
  - `#!/usr/bin/env --split-string=awk -f`
  - `#!/usr/bin/env -S -i awk -f`
  - `#!/usr/bin/env -S VAR= awk -f`
So update the current detection logic to detect those cases.

closes: #17199

Signed-off-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>