feat: add code-snippets for content creation (#206)

* Create astro-paper.code-snippets

* feat: allow nullable modDatetime

this is needed to allow the snippet to work with a blank date time, and is a precursor of adding in a modified date time setting hook

* Update astro-paper.code-snippets to make feat false by default

src/content/config.ts

@@ -7,7 +7,7 @@ const blog = defineCollection({
     z.object({
       author: z.string().default(SITE.author),
       pubDatetime: z.date(),
-      modDatetime: z.date().optional(),
+      modDatetime: z.date().optional().nullable(),
       title: z.string(),
       featured: z.boolean().optional(),
       draft: z.boolean().optional(),