Add snackbar util

src/lib/util/snackbar.ts

@@ -0,0 +1,18 @@
+import { writable } from "svelte/store";
+
+type Snackbar = {
+  visible: boolean;
+  message: string;
+};
+export const snackbarStore = writable<Snackbar | undefined>(undefined);
+
+export function showSnackbar(message: string, duration = 3000) {
+  snackbarStore.set({
+    visible: true,
+    message
+  });
+
+  setTimeout(() => {
+    snackbarStore.set(undefined);
+  }, duration);
+}