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]
+}

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]
+}

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]
+}

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]
+}