Create getPostsByTag util function

src/utils/getPostsByTag.ts

@@ -0,0 +1,8 @@
+import type { MarkdownInstance } from "astro";
+import { slufigyAll } from "./slugify";
+import type { Frontmatter } from "./types";
+
+const getPostsByTag = (posts: MarkdownInstance<Frontmatter>[], tag: string) =>
+  posts.filter((post) => slufigyAll(post.frontmatter.tags).includes(tag));
+
+export default getPostsByTag;