Use `vs-pwsh` icons if applicable. (#19990)

## Summary of the Pull Request
This PR updates `VsDevShellGenerator` to use the `vs-pwsh` icon in
generated profiles, if modern PowerShell has been detected.

## References and Relevant Issues
The icons were added in #17706, but are not used anywhere.

## Detailed Description of the Pull Request / Additional comments
* Updated `VsDevShellGenerator::GetProfileCommandLine` to accept a
`bool& isPwsh` parameter, which is set to whether the generated profile
command line is using modern PowerShell. This value gets passed to
`VsDevShellGenerator::GetProfileIconPath`'s new parameter, which
determines whether to return the icon for `powershell` or `pwsh`.