Add ios16 compatibility and improve scrolling handling

[borsosbe]

• change MarkdownUI final class to struct because of swiftUI (iOS 16) add WKNavigationDelegate didFinishNavigation
• delegate to handle show markdown flow, it is cleaner and more reliable than waiting 0.3 seconds on MarkdownView. there were cases when evaluateJavaScript() threw errors because of a too-early call
• change isScrollingEnabled to true, now it only starts to scroll horizontally when the content is larger, vertical scrolling not affected by this so it's more convenient that way