refactor: Rewrite diff logic to actually return differences, not just breaking changes

[pawamoy]

Is your feature request related to a problem? Please describe.

I have plans for Griffe 😤 I want not only to detect breaking changes, but all kinds of things: places where the API can be improved, places where the API is not super forward-compatible, things that became deprecated, etc.. I want to support adding these "Changed in", "Removed in", "Added in" admonitions in docs automatically.

Currently the diff module only yields breaking changes. Adding other utils to yield plain changes would duplicate the code and make things inefficient.

Describe the solution you'd like

Rewrite utils to yield plain changes. Each change could then have flags? Like a "hint", "warning", "deprecation", "breakage" flag or flags. The find_breaking_changes function would simply filter the detected changes.

Describe alternatives you've considered

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Additional context

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