feat: make the Theme type support Swift 6 (#351)

Author: MojtabaHs

Sources/MarkdownUI/Theme/Theme.swift

@@ -99,7 +99,7 @@ import SwiftUI
 ///   }
 ///   // More block styles...
 /// ```
-public struct Theme {
+public struct Theme: Sendable {
   /// The default text style.
   public var text: TextStyle = EmptyTextStyle()