Improving the discoverability of PowerShell variables #4394

Continue #4216 to discuss improving the discoverability of PowerShell variables.

Problem: It is hard for users to discover PowerShell automatic (system) variables and get help (their description).

Idias:

1. Enhance cmdlets to provide better discoverability, tab-completion, etc.
   1.1. Get-Variable -AutomaticVariable/-PowerShell/-System/-Special / -PreferenceVariable
   1.2 Get-Help -AutomaticVariable/-SpecialVariable / -PreferenceVariable
2. Add new namespace $pspref or $psvar
3. Enhance the variable: provider with additional properties IsAutomatic and IsPreference.