Make some basic timer functionality

src/App.svelte

@@ -1,5 +1,26 @@
 <script>
-  let seconds = 5;
+  import { onDestroy } from 'svelte'
+  import { invoke } from '@tauri-apps/api'
+  import { listen, once } from '@tauri-apps/api/event'
+  
+  let seconds = 5
+
+  function start_timer() {
+    invoke('start_timer', {
+      duration: { secs: seconds, nanos: 0 },
+      message: "Hi!",
+    })
+
+    let timer_tick_unlisten = listen('timer-tick', (event) => {
+      console.log("Tick!", event.payload.id, event.payload.elapsed)
+    })
+
+    once('timer-done', (event) => {
+      console.log("Done!", event.payload.id)
+
+      timer_tick_unlisten.then(ttu => ttu())
+    })
+  }
 </script>
 
 <main>
@@ -7,5 +28,5 @@
     Fire after
     <input type="number" bind:value={seconds} />
   </label>
-  <button>Fire!</button> 
+  <button on:click="{start_timer}">Fire!</button>
 </main>