Debugging: tracking issue for MVP and post-MVP work

[cfallin]

The "MVP release" of the user-facing guest-debugging feature in Wasmtime is pretty close now, and requires:

• Debugging: add the debug-main world. #12756: the environment in which debug components run
• Debugging: add builtin gdbstub component. #12771: the built-in gdbstub component, usable via wasmtime run -g
• Build libxml2 for LLDB WebAssembly/wasi-sdk#614 and corresponding release: a release of wasi-sdk with LLDB containing all necessary features
• update documentation to describe the guest-debugging flow

Post-MVP we have the following functionality queued up in some order:

• Debugging: implement debug component support for wasmtime serve #12776: debugging under wasmtime serve
• Debugging: implement memory watchpoints #12188: memory watchpoints
• "mutable debugging": the ability to inject calls, force early returns, and update locals/operand-stack values
  • several levels here; the last two don't have gdbstub commands and aren't really necessary to change source-level locals in debug/non-opt Wasm component builds
• record/replay and debugging of replayed traces (cc @arjunr2)
• Store snapshots and reversible debugging
• debug-adapter DAP world, top-level DAP multiplexer, and adapter components for DWARF and for an interpreter