Local untracked config overrides (prek.local.toml)

I'm trying to introduce shared pre-commit hooks to a shared existing monorepo. We're hitting problems where some users want to run quite a lot of checking as part of their pre-commit hooks, and others want the bare minimum. Ideally we'd like to have some base prek config that is checked in and shared, while allowing users to add their own config on and have this stay local to their checkout.

The way some other tools like (mise)[https://mise.jdx.dev/configuration.html#mise-toml] handle this is to recognise a mise.toml and a mise.local.toml, and at run-time merge them together with the local file taking precedence. Presumably for prek this could use the same or similar semantics to workspace mode.

If you're open to this, I'd be happy to put together a PR.