Detect unused protected methods

[staabm]

POC for #143

[staabm]

I intentionally kept the PR to a very minimum to see whats actually needed.
in case we agree on this change, I would rename a few classes, e.g.
PublicClassMethodMatcher -> UnusedClassMethodMatcher

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I will run this POC on a real world codebase in the mid of the week to get a better idea whether we are missing something important

[staabm]

@TomasVotruba I think this will work. how could you imagine support for protected methods?

do we need separate rules and collectors for protected stuff or can we change "public" into "not-private"?
would we need separate feature switches in the phpstan config file to enable "protected" analsis?

(I would be fine with only supporting protected methods. I don't need support for protected properties. IMO methods are way more valuable then constants or properties as they contain way more code)

[TomasVotruba]

Not sure this is worth it, as provided method are private as in final class. Coincidentally wrote about it here :)
https://tomasvotruba.com/blog/why-final-classes-make-rector-and-phpstan-more-powerful

I'd prefer to keep this package simple, focus on public only and let final handle the private. There PHPStan detect unused methods out of the box.