Use readonly properties instead of public fields

src/Avalonia.Visuals/Media/Fonts/FontKey.cs

@@ -4,10 +4,6 @@ namespace Avalonia.Media.Fonts
 {
     public readonly struct FontKey : IEquatable<FontKey>
     {
-        public readonly string FamilyName;
-        public readonly FontStyle Style;
-        public readonly FontWeight Weight;
-
         public FontKey(string familyName, FontWeight weight, FontStyle style)
         {
             FamilyName = familyName;
@@ -15,6 +11,10 @@ namespace Avalonia.Media.Fonts
             Weight = weight;
         }
 
+        public string FamilyName { get; }
+        public FontStyle Style { get; }
+        public FontWeight Weight { get; }
+
         public override int GetHashCode()
         {
             var hash = FamilyName.GetHashCode();