Enable shell detection for Windows

[stevapple]

The conventional SHELL environment variable is set by OpenSSH server on Windows, but we need to normalize the path slightly differently. Some popular shell names on Windows are cmd (Command Prompt), powershell (Windows Powershell), pwsh (Powershell Core) and bash (Git Bash).

Checklist

• [ ] I've added at least one test that validates that my change is working, if appropriate
• [x] I've followed the code style of the rest of the project
• [x] I've read the Contribution Guidelines
• [x] I've updated the documentation if necessary

[natecook1000]

Thanks, @stevapple! Do you know if the Bash completion scripts that ArgumentParser creates work on Windows?

[compnerd]

@natecook1000 no, they would not. There is no completion support for cmd.

[natecook1000]

Since ArgumentParser's shell detection is only to support the completion script request, it doesn't sound like we need this change.