Add new API for PageView implementation in SwiftUI

[rechsteiner]

The new API replaces the PagingItem initializer with a new Page struct that let's you define custom header items per page. Each page can define their own custom SwiftUI view as the menu item, and it can be customized based on the selection and progress. The new API also supports result builders, which makes it easier to create PageViews with a fixed number of pages.

PageView {
    Page("Title 0") {
        Text("Page 0")
    }
    Page("Title 1") {
        Text("Page 1")
    }
}