Make rubocop-packs work as a RuboCop plugin

[jeromedalbert]

Resolves #85.

This PR adds support for RuboCop's Plugin feature: rubocop/rubocop#13792

[tim-gq]

@jeromedalbert Would love to see this. Do your changes prevent it from working with older versions of rubocop, or is this backwards compatible?

[jeromedalbert]

This is backwards compatible. People can use require in their rubocop.yml and it will still work, but a warning will be emitted.

[dduugg]

Thanks for this! The migration from the old Inject monkey-patch to LintRoller::Plugin is a clean improvement and the implementation looks correct.

One minor nit: lib/rubocop/packs/plugin.rb uses # typed: false while the rest of the codebase uses typed: strict. Happy to take that as a follow-up if it's not straightforward to satisfy Sorbet here.

[jeromedalbert]

If I remember correctly I had issues with typed: strict so I used # typed: false as a quick win. But I don't use Sorbet, so feel free to go ahead and improve on this!