Add Svelte 5 Support

Modifying script loading to accommodate the upcoming release of Svelte 5.
The `on:load` event for scripts placed inside `<svelte:head>` does not
fire in Svelte 5 as a result of this bug:
sveltejs/svelte#8301

Instead of loading the Cloudfare Turnstile script in `<svelte:head>`, load
the script dynamically during mount. This is functionally equivalent to
using `<svelte:head>` but accommodates an operational load event handler.
If Svelte 5 resolves Issue 8301, this is not needed.

Author: mstachowiak

src/lib/Turnstile.svelte

@@ -34,6 +34,16 @@
     onMount(() => {
         mounted = true;
 
+        if (!loaded) {
+            let script = document.createElement('script');
+            script.type = 'text/javascript';
+            script.src =
+                'https://challenges.cloudflare.com/turnstile/v0/api.js?render=explicit';
+            script.async = true;
+            script.addEventListener('load', loadCallback);
+            document.head.appendChild(script);
+        }
+
         return () => {
             mounted = false;
         };
@@ -101,15 +111,6 @@
     };
 </script>
 
-<svelte:head>
-    {#if mounted && !loaded}
-        <script
-            src="https://challenges.cloudflare.com/turnstile/v0/api.js?render=explicit"
-            on:load={loadCallback}
-            async></script>
-    {/if}
-</svelte:head>
-
 {#if loaded && mounted}
     {#key $$props}
         <div use:turnstile />