feat: define blog schema and add blog collection

src/content/_schemas.ts

@@ -0,0 +1,20 @@
+import { z } from "astro:content";
+
+export const blogSchema = z.object({
+  author: z.string(),
+  pubDatetime: z.date(), // z.string().datetime() only available in Zod v3.20.2
+  title: z.string(),
+  postSlug: z.string().optional(),
+  featured: z.boolean(),
+  draft: z.boolean(),
+  tags: z.array(z.string()),
+  ogImage: z
+    .object({
+      src: z.string(),
+      alt: z.string().optional(),
+    })
+    .optional(),
+  description: z.string(),
+});
+
+export type BlogFrontmatter = z.infer<typeof blogSchema>;