Create `skeleton()` modifier

[PavelHolec]

The modifier should behave like the native redacted() modifier (iOS14+ only, otherwise we would simply use that one). Its purpose is to redact/skeletonize the content inside components and replace it with appropriate skeleton shapes (recursively, for all subcomponents).

If possible, the iOS14+ redactionReasons environment should be used and supported, so that the skeleton() acts as a iOS13 wrapper.

The easiler alternative is to purely use the redacted() modifier and support this enhancement only for iOS14+.

Atomic components would act upon this environment value and use appropriate skeleton overlay.

Intermediate components could override this and create a specific skeleton overlay for customized parts.

This makes it easier to "skeletonize" the whole ListChoice etc. based on actual (mocked) data.