fix: Expose which accessibility selectors are actually allowed for a particular node

[mwcampbell]

I think this is a good thing to add before implementing the NSAccessibility text methods, which an AT shouldn't try to call on nodes that don't support text ranges. Since I've chosen to have a single Objective-C class for all types of AccessKit nodes, it's useful to let ATs know which methods are and are not supported for a particular node.

I know this method is being used, because before I added accessibilityRoleDescription to the list of selectors that are allowed by default, VoiceOver didn't speak some role descriptions, like "slider", "stepper", and "button". Maybe we'll eventually discover more selectors that need to be on the default allow list.

[DataTriny]

I'll trust you on this one.

[rain-sk]

Have you checked if this works in practice?

In my experience, supporting isAccessibilitySelectorAllowed:@selector(accessibilityRoleDescription) did not actually work as desired. It seemed to cause problems with how VoiceOver read the roles of elements in an accessibility tree.

The only way I've found to properly support custom role descriptions is to not implement accessibilityRoleDescription and instead use setAccessibilityRoleDescription on the NSAccessibilityElement* if a node actually has a custom role description.

[rain-sk]

In general, I've found VoiceOver's use of isAccessibilitySelectorAllowed to be inconsistent, and not reliable. It's why the direction I took was to define separate sub-interfaces of NSAccessibilityElement<NSAccessibilityElement>.

[mwcampbell]

I found that overriding accessibilityRoleDescription is working fine so far. I'll be submitting support for custom role descriptions as a separate PR. The trick, I think, is to call the method on the superclass if we don't have our own role description.

Still, I guess I should wait and merge this PR only once I've confirmed that this approach works for the text-related selectors.

[mwcampbell]

So far, my current approach, having a single node class and implementing isAccessibilitySelectorAllowed:, is working well, even after introducing text editing, so I'll merge this PR now.