Built in Debugger: Rework interfaces, make sure things work

[skmp]

Currently the Visual Debugger is non functional, and it depends on debug interfaces that are no longer implemented and miss features like multiple thread support.

A large part of those interfaces was disabled in #1842.

We need to

Milestone 2210

• Define a fuller, actually implemented debug interface
• Add tests for that interface

Milestone 2212

• Update the visual debugger & gdb shims to use those interfaces, or make a decision to remove the visual debugger / gdb shim
• If we keep the visual debugger / gdb shims, make sure they are built and tested.

Follow up from #1842