Add support for command input type in launch.json

[mltoo]

See #1082.

[mfussenegger]

I'm not yet sure how to deal with this.

It would be great if at least some of the basics just work out of the box, but looking at what kind of commands exist it seems that that's basically everything one can do in vscode. That's not feasible to include and including a subset would make it harder to draw a clear line - although not impossible.

So making this extensible seems like a good call, but I think I'd rather go in a even more general direction to also account for #720
That means some sort of extensible configuration pre-processing mechanism