Create a dummy value by default for LocalSnackbarDispatcher. (#715)

This should solve the issue with needing to provide a `LocalSnackbarDispatcher` for previews and screenshot tests.

libraries/designsystem/src/main/kotlin/io/element/android/libraries/designsystem/utils/Snackbar.kt

@@ -59,9 +59,7 @@ class SnackbarDispatcher {
 }
 
 /** Used to provide a [SnackbarDispatcher] to composable functions, it's needed for [rememberSnackbarHostState]. */
-val LocalSnackbarDispatcher = compositionLocalOf<SnackbarDispatcher> {
-    error("No SnackbarDispatcher provided")
-}
+val LocalSnackbarDispatcher = compositionLocalOf<SnackbarDispatcher> { SnackbarDispatcher() }
 
 @Composable
 fun SnackbarDispatcher.collectSnackbarMessageAsState(): State<SnackbarMessage?> {