Fix bullets in visionOS (#308)

Label does not align the icon with the center of the first line like on iOS

Signed-off-by: Antonio Cabezuelo Vivo <antonio@tapsandswipes.com>

Author: tapsandswipes

Sources/MarkdownUI/Views/Blocks/ListItemView.swift

@@ -40,5 +40,30 @@ struct ListItemView: View {
         .readWidth(column: 0)
         .frame(width: self.markerWidth, alignment: .trailing)
     }
+    #if os(visionOS)
+    .labelStyle(BulletItemStyle())
+    #endif
   }
 }
+
+
+extension VerticalAlignment {
+   private enum CenterOfFirstLine: AlignmentID {
+      static func defaultValue(in context: ViewDimensions) -> CGFloat {
+         let heightAfterFirstLine = context[.lastTextBaseline] - context[.firstTextBaseline]
+         let heightOfFirstLine = context.height - heightAfterFirstLine
+         return heightOfFirstLine / 2
+      }
+   }
+   static let centerOfFirstLine = Self(CenterOfFirstLine.self)
+}
+
+
+struct BulletItemStyle: LabelStyle {
+    func makeBody(configuration: Configuration) -> some View {
+        HStack(alignment: .centerOfFirstLine, spacing: 4) {
+            configuration.icon
+            configuration.title
+        }
+    }
+}