Show feedback when code is copied

src/lib/PreCopy.svelte

@@ -1,12 +1,14 @@
 <script>
     let slot
-    function copy_code() {
-        navigator.clipboard.writeText(slot?.innerText)
+    let copied = false
+    async function copy_code() {
+        await navigator.clipboard.writeText(slot?.innerText)
+        copied = true
     }
 </script>
 
 <pre>
-<button on:click={copy_code}>Copy code</button>
+<button on:click={copy_code}>Copy code</button> {#if copied}<span style="display: inline">Copied!</span>{/if}
 <span bind:this={slot}><slot></slot></span>
 </pre>