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/1_users.dbml

@@ -0,0 +1,5 @@
+// First file - users table basic structure
+Table public.users {
+  id bigint [pk, increment]
+  email varchar(255) [unique, not null]
+}