Allow adjustment for safe area insets

[tbrushwyler]

This PR addresses Issue #4, allowing adjustments based on the safe area insets. This expands on the default implementation provided for iOS devices, allowing more elements to be adjusted than just the page.

One use case is a ScrollView with content taller than the page. Instead of adding padding to the ScrollView's container (page), we can add the padding to the ScrollView itself. This gives a more natural look and feel to the page.

Android handles cutouts differently, so we don't need to do anything special.

[tbrushwyler]

To use these values, you can either reference an inset directly (e.g. Double_SafeAreaInset_L) or reference a thickness with a combination of the insets (e.g. Thickness_SafeAreaInsets_T, Thickness_SafeAreaInsets_H, Thickness_SafeAreaInsets_LRB).