Fix post sorting (#4)

Fix post sorting.
- sort posts by name
- posts are named by date : 20231126.md

Reviewed-on: #4

main.cr

@@ -96,7 +96,7 @@ class PostRepository
   end
 
   def all : Array(Post)
-    posts = Dir.glob("#{@config.content_dir}/*.md").map do |file|
+    posts = Dir.glob("#{@config.content_dir}/*.md").sort.map do |file|
       Post.new(file)
     end
     posts.reverse