Provide a lightweight shell selector without changing default

[Tyriar]

I built the https://github.com/Tyriar/vscode-shell-launcher extension which allows the user to create profiles to launch, this is particularly useful on Windows where the user is much more likely to use multiple shells. We have a similar feature in with the default shell selector:

The issue is that it changes the default setting. It can also only change the shell, not shell args currently which makes supporting multiple WSL distro not possible (#77652).

We should add a feature similar to Windows Terminal where current shells are detected and it also enables users to setup their own profiles including shell args.

[gregvanl]

Doc feedback verbatim 8/6/2019: "How do I have both an integrated powershell and integrated command prompt and maybe others open at same time? It only lets me start up a terminal based on the default? So would I just have to change the default all the time? What am I missing?"

[Tyriar]

/duplicate #45444