Support sizing labels accounting for their cap insets

We already support aligning and distributing labels taking into account their cap insets, but sizing currently requires the _entire_ available space. We should add equivalent APIs for sizing:

```swift
func capInsetSizeThatFits(_ size: CGSize, constraints: SizingConstraints = .none) -> CGSize

func capInsetSizeToFit(_ size: CGSize, constraints: SizingConstraints = .none)
```