Add support for PowerShell completion so TerraForm and other library consumers can inherit that support

[nam20485]

Terraform cannot include PowerShell autocompletion sort because they use your library and they believe your library does not offer support for PowerShell completion.

From TerraForm GitHub issue:

Terraform's tab-completion is powered by the external library github.com/posener/complete, so we support the same set of shells which that library supports. Therefore, if you send this request to that library or propose a PR that they accept then we can upgrade our version of that library to inherit that PowerShell support.

Apparently a rival completion library called Cobra does offer support for PowerShell.