feat: 添加代码片段加载页面并实现延迟加载

src/app/snippets/[id]/loading.tsx

@@ -0,0 +1,3 @@
+export default function SnippetShowLoadingPage() {
+  return <div>Loading...</div>;
+}

src/app/snippets/[id]/page.tsx

@@ -8,6 +8,8 @@ interface SnippetShowPageProps {
 }
 
 export default async function SnippetShowPage(props: SnippetShowPageProps) {
+  await new Promise((r) => setTimeout(r, 2000));
+
   const params = await props.params;
   const snippet = await db.snippet.findFirst({
     where: {