Behavior of `Set-EnvironmentVariable` contravenes creation-on-demand expectations

Long established `Set-*` cmdlets such as `Set-Variable` and `Set-Alias` have _creation on demand_ logic.

For instance, `Set-Variable NoSuchVarYet NewValue`  creates `$NoSuchVarYet` _if it doesn't exist yet_.

By contrast, `Set-EnvironmentVariable` only succeeds if the target environment variable already exists.
