Remove prohibition on mixing snake_case and camelCase at least for constructors

+1 for moving this forward as @xjm describes in #15

Attempting a re-title, but this also needs an issue summary update.

@catch what about the public static function create() {...}? I think it falls in the same category but the proposed title doesn't cover it.

I think we have essentially done this issue in #2648050: [policy, no patch] Stop disallowing camelCase for local variables / parameters → the way that worked out - i.e. we're just allowing either style for local variables now. Is there anything else to do here?

@catch, I think this part from standards is the problem

Be consistent, do not mix camelCase and snake_case variables inside a method or function.

A constructor extending the parent (which have snake_case params) cannot add a new param with constructor property promotion because that is camelCase. So, it will mix snake_case with camelCase, violating the quoted text

@claudiu.cristea OK that makes sense, let's reword or remove that. Updated the issue title for now.