VSCode: Add Cortex Debug launch config to attach to a running target.

[microbit-carlos]

@JohnVidler This might also be helpful when debugging CODAL and/or pxt C++ code with MakeCode projects (the TS user code doesn't have symbols for the debugger, but you can still inspect the C++ stack trace or add breakpoint to the C++ sources).

[JohnVidler]

Looks good to me, lets merge this in :)