Backend rewrite for v1.0

[jayqi]

This is a major rewrite of erdantic's backend for v1.0. See HISTORY.md file for summary of changes.

Issues addressed:

• Closes Support for attrs #9
  • Added!
• Closes Use optional dependencies for integrations with frameworks? #17
  • We're adding support for attrs, and attrs is now an optional dependency. Pydantic is not because we use Pydantic ourselves now for the backend data models.
• Closes Add extra Column #70
  • Docs page "Extending and modifying" shows an example of adding an additional column under the new design
• Closes Feature: Customizable graph label -> remove erdantic watermark #78
  • This can be accomplished by passing in a graph_attr={"label": "..."} override.
• Closes Allow user to provide custom cardinality and modality #88 and Closes Create edges with modality and cardinality on left and right-hand side #91
  • This can be accomplished by modifying target_cardinality, target_modality, source_cardinality, and source_modality fields on Edge
• Closes Modality zero should only be determined by Optional typing #90
  • Collections of models like List[SomeModel] now treat modality as unspecified.
• Closes Cardinality incorrect with typing.Optional typing.List for Pydantic dataclasses? #105
  • Now correctly parses the full type annotation tree to determine manyness.