superbol-vscode-debug: bug: `type: "gdb"` overrides system debugger

[GitMensch]

The problem is that with the current type, I cannot debug native C programs any more, because "gdb" conflicts with the native.debug extension.

I suggest to use cob-gdb or something else instead.

[nberth]

Note: with changes in https://github.com/OCamlPro/superbol-vscode-debug/pull/8, selecting Start Debugging from the menu with no launch.json does nothing (previously that started a debug session). I'm not sure what is causing that.