Multiple shells / Shell detection

[mxsdev]

Currently, we try to determine what shell to execute in based on the SHELL environment variable. This causes a few issues:

• We cannot have different scripts execute in different shells, e.g. based on the language of the fenced code block
• SHELL is not necessarily set on Windows, so this is a significant hurdle for windows support

VSCode internally has a list of TerminalProviders which can be provided by extensions. We should:

• Tap into this if possible
• If not, develop a solution to detect shells on the host's system, ideally leveraging existing solutions/libraries

With this information, we can:

• Give user the option to select a shell profile to run a command block in
• Try to make a best guess for which shell to use by default (using language, user's preferred shell, etc)
• Use gRPC runner on Windows

For windows support, runme will need to implement support for powershell and DOS. This should not be that difficult, see stateful/runme#173