Small caps support in iOS?

### What is the problem or limitation you are having?

While poking around in the font backends, I noticed that while macOS checks for small caps:

https://github.com/beeware/toga/blob/a4f6b95e1704f711e87f6a63561a074c7c77e976/cocoa/src/toga_cocoa/fonts.py#L110-L123

iOS doesn't:

https://github.com/beeware/toga/blob/a4f6b95e1704f711e87f6a63561a074c7c77e976/iOS/src/toga_iOS/fonts.py#L109-L120

And it's ignored in test probe as well.

https://github.com/beeware/toga/blob/a4f6b95e1704f711e87f6a63561a074c7c77e976/iOS/tests_backend/fonts.py#L38-L49

I see from the docs that there is indeed no corresponding `UIFontDescriptorSmallCaps`. Although there is [`kLowerCaseType`](https://developer.apple.com/documentation/coretext/klowercasetype?language=objc). Is it possible to use that to support small caps?

### Describe the solution you'd like

Ideally, we'd support small caps on every (possible) platform.

### Describe alternatives you've considered

Leave it as is, I suppose.

### Additional context

I don't understand much about the macOS or iOS font systems (yet...), but some [cursory searching](https://stackoverflow.com/questions/12941984/typesetting-a-font-in-small-caps-on-ios) indicates that, at least for fonts that support it, small caps is possible on iOS.

Compare the small-caps handling in cocoa/src/toga_cocoa/fonts.py with iOS/src/toga_iOS/fonts.py, then inspect iOS/tests_backend/fonts.py and the linked CoreText documentation. Determine whether kLowerCaseType can provide the requested behavior on iOS, and update the backend and font test probe if it can. Done means iOS supports small caps where possible and the probe covers the behavior.

Written by the indexing model from the issue text.