Add macos detection to New-CommandDataFile

[GavinEke]

PR Summary

Add MacOS Detection as $os in PSCore v6.0.0-beta.7 and higher are getting incorrectly set as windows in the Get-CmdletDataFileName function

More details: https://github.com/PowerShell/PowerShell/blob/master/docs/BREAKINGCHANGES.md#rename-isosx-to-ismacos-4700

PR Checklist

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• [NA] User facing documentation needed
• Change is not breaking
• [NA] Make sure you've added a new test if existing tests do not effectively test the code changed
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[bergmeister]

Thanks for contributing. The change makes sense to me. But I do not see the need to support old Beta versions of PowerShell Core, therefore I removed the elseif ($IsOSX) block. Otherwise looks good to me. I tend to keep PRs open for at least 24h for visibility to others and will merge it then if there are no objections from others.