feat: Text support on macOS

[mwcampbell]

This should be on par with the Windows adapter, meaning support for single-line and multi-line edit controls, but without exposing text formatting attributes or other rich text features. To test with a real toolkit implementation, use this egui branch. egui itself required only one minor fix, which is already merged; the existing integration in egui that was tested with the Windows adapter was mostly fine already.

[mwcampbell]

I'm aware that the new functions for converting to and from indices are O(n), where n is the number of lines or even characters. I know this won't scale to huge chunks of text, but I've decided not to bother with caching or the like until we know that we need it. After all, cache invalidation is one of the hardest problems in computer science.

[trypsynth]

I don't even think AppKit bothers with caching as far as that goes, because when in a large text field, pressing command+down to try to jump to the end causes the application to lock up for a good while. If not lock up, at the very least, make VoiceOver think it went not responding. I'm unsure if it also blows up for sighted users, but I doubt it.
This freeze period is longer on Intel Macs, but still happens on M1's. If this proves to be a problem you can look into it, but I wouldn't stress it.

[DataTriny]

Sounds good to me, but I'd prefer if someone with MacOS development knowledge could have a look.

[mwcampbell]

@spencerudnick Can you please review this one?

[rain-sk]

Nice work, everything looks good to me. Though I had one question about claiming to support setAccessibilityValue: without actually implementing it.