1. Use 'canimport' flag to organize the codes related to UIKit.
   (If UIKit is not available, AppKit determined that it is available and wrote the code.)
2. WKWebView on Mac does not have UIScrollView, unlike WKWebView on iOS. Therefore, we created and used 'ScrollAdjustedWKWebView', which overrides the 'scrollWheel' function so that the surrounding UIScrollView can be scrolled.
3. Mac does not offer the SFSafariView Controller. Therefore, do not support .sfsafariview in an environment without UIKit.Use Safari as the default for link selection.

PS. MessageUI deleted the import code because it appears to be an unused framework. You can rewrite it when you want.

related issue link: #34