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/3_add_columns.dbml

@@ -0,0 +1,5 @@
+// Third file - adds more columns to users table (tests merging)
+Table public.users {
+  name varchar(100)
+  created_at timestamp [not null]
+}