Implemented context menu separators

[d2dyno1]

This PR adds context menu separators on all platforms.

• Added separators for Android, iOS, macOS, and Windows
• On Android SDK < 28, a button option appears with text content set to underline characters to simulate a separator
• Upgraded to .NET 10
• Added BaseContextMenuItem base for ContextMenuItem and ContextMenuSeparator which will make it easier to extend in the future
• Changed some != null and == null to is not null and is null
• Changed new UIImage(contextItem.Icon.File) to UIImage.FromBundle(contextItem.Icon.File)
• Bumped iOS/MacCatalyst minimum version from 14.0 to 15.0
• Improved code formatting in some places

iOS

MacOS

Android

SDK >= 28

SDK < 28

Windows

No screenshots. The code should be working anyway.

[d2dyno1]

I'd need some help from you. I didn't write tests for my changes, nor have I run any existing ones. Could you do that? Also, I need your opinion on the Android SDK < 28 implementation - alternatively, the separator could be completely ignored in this case. Personally, I think Android 5 (SDK 21) is quite old now, and should be removed. What do you think?

@anpin