feat(nodes): Node Groups

[psychedelicious]

Summary

Many node based tools allow users to group nodes together to be able to improve workflow visibility and simplify reusing parts of workflows

Intended Outcome

• Users are able to group and save nodes together in InvokeAI's workflow editor
• These groups are able to be reused across workflows

There's some support for this in reactflow.

This probably needs to be represented very generally in the workflow objects, and derived by the app. This way we can change the implementation without breaking existing workflows.

Requires careful planning.

[JPBarringer]

@psychedelicious would this be different from embedded workflows (workflows-as-nodes)?