implement call stack (also in embedded debugger)

[floooh]

Nice to have! This would involve tracking subroutine call instructions in the embedded debugger and tracking their call targets.

And then:

• [ ] add a new window to the embedded debugger with the callstack (clicking on the callstack should move the debug window there so that it's possible to set a breakpoint
• [ ] add callstack support to the emulator's webapi
• [ ] ...and implement the call stack for the vscode debug adapter.