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/9_refs.dbml

@@ -0,0 +1,10 @@
+// File with commented-out refs - should load last
+// Contains relationships that depend on earlier tables
+
+// Ref: public.posts.user_id > public.users.id [ondelete: CASCADE]
+
+Table public.comments {
+  id bigint [pk, increment]
+  post_id bigint [not null]
+  content text [not null]
+}