Must escape single quotes in fish `shellCommand` completions

[rgoldberg]

Must escape single quotes in fish shellCommand completions.

ArgumentParser version: main
Swift version:
swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx15.0

Checklist

• If possible, I've reproduced the issue using the main branch of this package
• I've searched for existing GitHub issues

Steps to Reproduce

Generate a fish completion script for a project that uses ' in a shellCommand completion.

Expected behavior

No problems.

Actual behavior

' aren't escaped, so the script breaks.