Improve Fish shell integration script

[kidonng]

Hi George, recently I created a standalone plugin based on the Fish integration script. I'd like to submit some of the changes back to iTerm2:

• Replace [ ] with test
• Replace sh -c "exit $last_status" with return $last_status
• Replace grep with Fish's string match builtin
• Replace wc with Fish's count builtin
• Simplify iterm2_has_fish_mode_prompt by directly using its return status

I hope you will like these improvements!