[breaking change]
Major changes are as follows:
- Add `Enable-ExperimentalFeature` and `Disable-ExperimentalFeature` cmdlets.
- Remove `-ListAvailable` from `Get-ExperimentalFeature`.
- Add `ArgumentCompleter` for `Get-ExperimentalFeature` cmdlet.
- Refactor some existing Experimental Feature tests.
- Make `ConfigScope` public and renamed `SystemWide` to `AllUsers`. Also update experimental feature code to prefer the current user config over the all user config.
