feat(reader): 🎉 Add support for multi-file DBML loading

* Implement directory reading for DBML files.
* Merge schemas and tables from multiple files.
* Add tests for multi-file loading and merging behavior.
* Enhance file discovery and sorting logic.

tests/assets/dbml/multifile/2_posts.dbml

@@ -0,0 +1,8 @@
+// Second file - posts table
+Table public.posts {
+  id bigint [pk, increment]
+  user_id bigint [not null]
+  title varchar(200) [not null]
+  content text
+  created_at timestamp [not null]
+}