Deprecate and ask for help with AppleSpell provider

[rrthomas]

The AppleSpell provider seems to be buggy these days (see e.g. #391), and I don't use macOS. I'd be happy to look into this on a paid consultancy basis; otherwise, I'd be delighted to have help from a macOS-using developer.

In the mean time, I am minded to deprecate the provider, and put it in the same category as the Zemberek back-end: not built by default, you get a warning if you enable it, and if it crashes that's a shame, but not my problem.