Simplify importing the default stub implementation of Shescape as
`Stubscape` by re-exporting it under that name.

As of writing, I'm not sure whether exporting this stub as `Shescape` or
`Stubscape` is better. I think both have pros and cons. In any case, the
export named `Shescape` can't just be removed because that would be a
breaking change.

Avoid vetting error by knip by marking `Stubscape` as an alias of
`Shescape` in its documentation. Besides making knip happy, this also
makes the re-export more explicit and hopefully clear in context.

To support this, `Shescape` was renamed to `Stubscape` and `Shescape`
now re-exports `Shescape`. This makes semantic sense (I want `Stubscape`
to be the "correct" export, and `Shescape` an alias for backwards
compatibility), but is also required as the (at)alias tag needs to be on
the re-exporting expression for knip to accept it.

Add `Stubscape` export to testing.d.ts and align documentation with
corresponding testing.js code.