Moving logic in executable target to framework

[lordcodes]

Currently, there is a lot of functionality built into the executable target 'swiftlint'. This means that if you want to use SwiftLint as a framework, such as when building a plugin for Tuist, most of whats in the executable target needs to be duplicated.

Would a PR be welcome that moves most of the CLI functionality, other than the ArgumentParser commands, to either SwiftLintFramework or to another framework in between the two?

This would allow SwiftLint to be used as a framework much more easily and would have no outside impact on users of the swiftlint executable. It would either be added as an extra layer of public API to the SwiftLintFramework or as a 2nd framework that sits between the two.

I am happy to work on the PR, but wanted to check it was a change that was actually welcomed first.