Support soft/purge uninstalling of scoop itself

[Ash258]

• Closes scoop uninstall scoop shouldn't remove persist data #2746

Why there is this code block?
https://github.com/lukesampson/scoop/blob/70bd6ae8c0f979924f85b9357a546859bd351553/libexec/scoop-uninstall.ps1#L26
I haven't found use case, which fires it.
- Should not it be here?
https://github.com/lukesampson/scoop/blob/70bd6ae8c0f979924f85b9357a546859bd351553/libexec/scoop-uninstall.ps1#L37

1. ENV in tested VM
   • Windows 10 PRO RS5 (1809 17763.1)
   • 
2. Scoop uninstall Total Commander
   1. Local
      1. Soft
         
      2. Purge
         
   2. Global
      1. Soft
         
      2. Purge
         
3. Scoop uninstall Scoop
   1. Local
      1. Soft
         
      2. Purge
         
   2. Global
      1. Soft
         
      2. Purge
         

[r15ch13]

https://github.com/lukesampson/scoop/blob/70bd6ae8c0f979924f85b9357a546859bd351553/libexec/scoop-uninstall.ps1#L26
This one checks if the apps parameter is set. scoop uninstall without an app name should trigger it.

https://github.com/lukesampson/scoop/blob/70bd6ae8c0f979924f85b9357a546859bd351553/libexec/scoop-uninstall.ps1#L37
This one checks if the result of ensure_all_installed() isn't empty.

Looks good to me. @rasa @brandon93s