feat: implement post search using Pagefind (#12)

src/pages/posts/[...slug].astro

@@ -52,6 +52,7 @@ const { remarkPluginFrontmatter } = await entry.render();
             <!-- title -->
             <div class="relative">
                 <div
+                    data-pagefind-body data-pagefind-weight="10" data-pagefind-meta="title"
                     class="transition w-full block font-bold mb-3
                     text-3xl md:text-[2.5rem]/[2.75rem]
                     text-black/90 dark:text-white/90