[AppKit/Foundation] Use Foundation's NSUnderlineStyle everywhere except for macOS until XAMCORE_4_0. (#12221)

src/AppKit/Enums.cs

@@ -1283,6 +1283,7 @@ namespace AppKit {
 		AsKeyedArchive = 4
 	}
 
+#if !XAMCORE_4_0 && MONOMAC // Use the one in Foundation instead, only keep this in macOS until XAMCORE_4_0.
 	[Native]
 	public enum NSUnderlineStyle : long {
 		None                = 0x00,
@@ -1296,6 +1297,7 @@ namespace AppKit {
 		PatternDashDotDot 	= 0x0400,
 		ByWord 				= 0x8000
 	}
+#endif
 
 	// Convenience enum, untyped in ObjC
 	public enum NSUnderlinePattern : int {

src/Foundation/Enum.cs

@@ -837,9 +837,9 @@ namespace Foundation  {
 		LongestEffectiveRangeNotRequired = 1 << 20
 	}
 
-#if !MONOMAC
-	// MonoMac AppKit redefines this
-	// NSInteger -> NSAttributedString.h
+#if XAMCORE_4_0 || !MONOMAC
+	// macOS has defined this in AppKit as well, but starting with XAMCORE_4_0 we're going
+	// to use this one only.
 	[Native]
 	public enum NSUnderlineStyle : long {
 		None	= 0x00,