Idea: support post-trap call traces for guest interpreters

While discussing this componentize-py issue, I realized it would be useful to allow a component to be entered one last time after it traps in order to get additional debug info.

For example, if a trap occurs while running Python code in the CPython interpreter, the guest stack trace produced by Wasmtime is usually pretty useless; what we really want is a Python-level stack trace, and that can only realistically be produced by executing guest code. More generally, it would be nice to be able to attach a Python-level debugger to the interpreter to do an interactive investigation; either way, we'd need to execute guest code.

One possible solution to this would be to allow the host to export a special on-trap function which the host may call after the trap happens, similar to how a signal handler would run after e.g. a SIGSEGV occurs. Presumably that function would be restricted from calling any imports and, in the simple case, take no parameters and return only a string containing a high-level call trace. Attaching a debugger would require a more sophisticated interface, of course, but we could start simple and expand from there.

Start by reviewing this proposal and the linked componentize-py issue 146; no source files, tests, or implementation entry points are named. Define and agree on the post-trap execution model, restrictions, return values, and debugger scope before implementation. Done means the Component Model behavior and its validation plan are specified.