Add crossfade transition (#286)

voyager-transitions/src/commonMain/kotlin/cafe/adriel/voyager/transitions/CrossfadeTransition.kt

@@ -0,0 +1,29 @@
+package cafe.adriel.voyager.transitions
+
+import androidx.compose.animation.Crossfade
+import androidx.compose.animation.core.FiniteAnimationSpec
+import androidx.compose.animation.core.tween
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Modifier
+import cafe.adriel.voyager.core.screen.Screen
+import cafe.adriel.voyager.navigator.Navigator
+
+@Composable
+public fun CrossfadeTransition(
+    navigator: Navigator,
+    animationSpec: FiniteAnimationSpec<Float> = tween(),
+    label: String = "Crossfade",
+    modifier: Modifier = Modifier,
+    content: @Composable (Screen) -> Unit = { it.Content() }
+) {
+    Crossfade(
+        targetState = navigator.lastItem,
+        animationSpec = animationSpec,
+        modifier = modifier,
+        label = label
+    ) { screen ->
+        navigator.saveableState("transition", screen) {
+            content(screen)
+        }
+    }
+}