Feature Request: Make project friendlier to build schemas

[Spikhalskiy]

1. For some reason, the classes that need to be used for the creation of schemas from scratch had been moved into an internal _classes package. This is not conventional. According to the doc, these classes originally were in a public package, which makes more sense: https://github.com/Vanderhoof/PyDBML/blob/6781ccfb1fc636d8076af09d2098c44609ce7dd0/docs/creating_schema.md

2. It would be great if main classes got a sort of copy() method that creates a detached instance from the parent entity, with all the main fields (and children) copied.
   For example, a copy() of Database should create a complete deep copy of the entities.
   At the same time, a copy() of a column should return a column with the same name, type, and notes. But detached from the parent table, like

def copy(self) -> Column:
    Column(
          name=self.name,
          type=self.type,
          unique=self.unique,
          not_null=self.not_null,
          pk=self.pk,
          autoinc=self.autoinc,
          default=self.default,
          note=self.note,
          comment=self.comment,
          properties=self.properties.copy()
    )