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Hello, Stu casually proposed yesterday adding a dedicated `unions` backend registration hook and I’m wondering which you all prefer?

```def rules():
  return [UnionRule(Linter, BanditLinter), UnionRule(Linter, BlackFormatter)]```
vs.

```def unions():
  return {Linter: [BanditLinter, BlackFormatter}```
<@UKPS08ZGW> and I don’t think one is obviously superior.

Some pros of `unions()`
- Less verbosity when registering multiple unions
- Dicts are more ergonomic than `UnionRule`

Some cons of `unions()`
- Suggests that you’re overriding the union, when really you are just augmenting it
- Possibly more boilerplate in `register.py` when we import each file and now have to call `my_file.rules()` and `my_file.unions()`