Move MarkdownPost to its own file

lib/DB/MarkdownPost.rakumod

@@ -0,0 +1,17 @@
+use v6.e.PREVIEW;
+
+use Pandoc;
+use JSON::Class:auth<zef:vrurg>;
+
+use DB::Post;
+
+#| A plain markdown post
+unit class MarkdownPost does Post is json;
+
+#| Marker for disambiguation between post types in json representation, the
+#| cheaty way
+has Bool:D $.markdown = True;
+
+method title(--> Str:D) {
+    markdown-title($!source)
+}