Support *nix platforms

[jwittner]

PowerShell Core is now cross-platform. We should investigate supporting the *nix world, including Mac.

Mac

• Find-UnitySetupInstaller
• Get-UnityProjectInstance
• Get-UnitySetupInstance
• Install-UnitySetupInstance
• Select-UnitySetupInstance
• Uninstall-UnitySetupInstance
• Start-UnityEditor
• ConvertTo-UnitySetupComponent
• Get-UnityLicense

Linux

• Find-UnitySetupInstaller
• Get-UnityProjectInstance
• Get-UnitySetupInstance
• Install-UnitySetupInstance
• Select-UnitySetupInstance
• Uninstall-UnitySetupInstance
• Start-UnityEditor
• ConvertTo-UnitySetupComponent
• Get-UnityLicense

[Ziugy]

To support command line installs for macOS I'm going to have to rework the flow of Install-UnitySetupInstance.

Steps need to be in this sequence

• Download installers (this just has to happen before running the installer if we want to parallelize installs and downloading.) Figured for the initial rewrite it would just go first.

• Sort order of installers by Unity version. (This needs to be done in case we have multiple versions of Unity and packages to install.)
• foreach Unity version
  • If macOS and previous Unity version installed, move back to /Applications/Unity/
  • Install main Unity setup installer first
  • Install all Unity components
  • If macOS, move installer from /Applications/Unity/ to /Applications/Unity-$version/

These changes are required because installer doesn't give an option to overload the install location.