Consider running knip as a peer

It may be useful to be able to run knip, but we really shouldn't pull it as a dependency.

Maybe we can check for its existence as a peer and run it if it exists?

I do also wonder if this would be a good point at which we expose the plugin architecture. So someone can have a "knip plugin" for the CLI.

Then we would just launch knip as part of the `analyze` command.

cc @webpro