Return the migration number (Unique ID or UID) for migration sorting.

Default behavior, by priority:

• The UID is generated by the class name if it ends with a number.
• If there are no numbers in the migration class, it tries to use the id in the filename.
• If not found, an exception is raised. Concrete migrations can override this method.

Example:

class MyMigration1234567 # << Order = 1234567 file db/1234567_my_migration.cr # << Order = 1234567