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

Default behavior (By order of priority):

• The uid will be generated by the class name, if the class name contains number at the end.
• If there's no numbers in the migration class, then it will try to use the id in the filename
• If not found, an exception is raised. This method can be overwritten by the concrete migration in case you need it.

Example:

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