Enumerate and run installed PowerShell versions

Currently we changed semantic of -version command line parameter from "run Windows PowerShell 2.0" to "show current PowerShell version".

Now we need a way to address a side-by-side scenario - to enumerate installed PowerShell versions and run needed PowerShell version.

[ ] Enumerate installed versions:

>powershell -versions

[ ] Run needed version:

powershell -versions <ver spec>

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