Add the ability to customize which packages' frames are folded

[nex3]

Right now, we have some hard-coded logic for folding stack chains that come from test errors. We fold core library frames and frames from the test, stack_trace, and stream_channel packages. We should make this set of packages customizable, at least by allowing users to add to it. The config for this could look something like:

folded_packages:
- pool
- async

I'm not super happy with that name—it only makes sense to people who know that "folding" is a thing that can be done to stack traces. But I also don't want to make it too verbose, and I'm having trouble coming up with a better alternative.

[kevmoo]

A thought: this will rarely be configured. Verbose is not a huge issue for something rarely used.

For rarely used features, discoverability is more important IMHO

[nex3]

@kevmoo Do you have a name suggestion?

[kevmoo]

stack_trace_folded_packages ?

[nex3]

I think the following would be a good format for this:

fold_stack_frames:
  only:
  - pool
  - async

# or
fold_stack_frames:
  except:
  - pool
  - async