Add Windows support for shell attach

[lostintangent]

This change updates the Docker: Attach Shell command in order to properly support Windows containers. Previously, this command assumed that it could run /bin/sh in the selected image, whereas this change will detect a Windows container, and run cmd instead.

Beyond this change, we should allow configuring the default shell use, so that Windows users could toggle PowerShell and Linux users could use Bash/etc.

[lostintangent]

Should this default to Powershell instead? I'd love to hear feedback here.

[heaths]

I keep hearing rumor (I have no inside knowledge one way or another..even though I couldn't share it anyway) that Windows is making the switch themselves, but I question whether that would be accepted in general. I think as long as this was an option for the extension, though, you could default to whatever you want.

[chrisdias]

here is the blog post: https://blogs.windows.com/windowsexperience/2016/11/17/announcing-windows-10-insider-preview-build-14971-for-pc/#hjIQ7kQcdSPrzb1z.97

Given this, i suggest we default to PowerShell instead of cmd.

[lostintangent]

@chrisdias Sounds good to me! I'll make this change.

[chrisdias]

@lostintangent all changes look good, but lets default to PS instead of CMD (see comment above).

[heaths]

@chrisdias understandable (glad you found the link; I remember hearing something along those lines), but still seems it should be a user option, because:

1. The terminal type for platforms is already an option for VSCode generally, and
2. Even people on Linux may not want sh, but bash csh, ec.

I am partial to PowerShell myself, but just thinking about what others may want. The default shell for Windows containers is cmd, so users may still want to default that.

[chrisdias]

@heaths I created new issue #60 to track making the shell configurable.

[lostintangent]

Merging this in and will address the shell configuration in a separate PR. Thanks for the review!