Implement rule deprecation mechanism

[Sija]

Refs #765

[Sija]

Using Deprecated annotation seems neat, yet while this works, I'm not happy about the fact that deprecated class warnings show up when running specs or building the binary, and I'm not sure yet how to resolve this issue.

@veelenga @nobodywasishere @straight-shoota WDYT?

[straight-shoota]

Yeah, it feels like this might be at the wrong layer. That's similar to the API docs for the rules, which don't serve as developer documentation, but user documentation.

The distinction here is that using these classes in the Ameba API is not (necessarily) deprecated.
But using the rules in your Ameba configuration is.

Excluding deprecation warnings with --exclude-warnings src/ would be too broad, and there's no fine-grained warning management.

I believe it would easier overall to not use @[Deprecated] annotation. What's the benefit of that anyway?
We could use a custom annotation (@[Ameba::DeprecatedRule]) which would achieve basically the same thing.

[Sija]

We could use a custom annotation (@[Ameba::DeprecatedRule]) which would achieve basically the same thing.

Good idea! I've updated the code with your suggestion.