Export either `Stubscape` or `Shescape` from `shescape/testing`

[ericcornelissen]

Caused by #1308

Summary

With the release of v3 the testing module should be simplified by removing the re-export Shescape and only exporting Stubscape (or Shescape if that name is preferred). Ideally, the one that will be removed is also marked as deprecated in a v2 release beforehand.

[ericcornelissen]

What we can do in v2 is deprecate one of the two