Consider a custom usethis hook for keeping config in-sync #1674

Sometimes we duplicate config e.g. src layout configuration for different tools.

Perhaps we could use tool.usethis as a source of truth for this, until PEPs land, and then we can force correspondence via pre-commit.

I think we could write a general hook which lets you designate multiple config file fields that should validate to be equal, or even have a source/target distinction to enable autofix.