appkit: mark makeCoordinator with MainActor.

* The appropriate method that should NOT have a main actor is
  determineViewSize, since that will break compiling with swift 6
  strict concurrency.

Signed-off-by: furby™ <[email protected]>

Author: furby-tm

Sources/AppKitBackend/NSViewRepresentable.swift

@@ -27,6 +27,7 @@ public protocol NSViewRepresentable: View where Content == Never {
     ///
     /// The coordinator is used when the view needs to communicate changes to the rest of
     /// the view hierarchy (i.e. through bindings), and is often the view's delegate.
+    @MainActor
     func makeCoordinator() -> Coordinator
 
     /// Compute the view's size.