fix: outer_variables crash

[katafrakt]

When calling Refactorex from Expert, I often see this error:

  11:22:06.870 instance_id=19E16560B72 [debug] Node port message:                                                                                                                               
  11:22:06.869 [error] Task #PID<0.5820.0> started from #PID<0.5806.0> terminating                                                                                                              
  ** (FunctionClauseError) no function clause matching in anonymous fn/1 in XPRefactorex.Refactor.Dataflow.outer_variables/1                                                                    
      (xp_refactorex 0.1.52) lib/refactorex/refactor/dataflow.ex:20: anonymous fn({:gen, {:env, [trailing_comments: [], leading_comments: [], line: 222, column: 9], nil}}) in                  
  XPRefactorex.Refactor.Dataflow.outer_variables/1                                                                                                                                              
      (elixir 1.19.4) lib/stream.ex:630: anonymous fn/4 in Stream.map/2                                                                                                                         
      (elixir 1.19.4) lib/enum.ex:5023: Enumerable.List.reduce/3                                                                                                                                
      (elixir 1.19.4) lib/stream.ex:1773: Enumerable.Stream.do_each/4                                                                                                                           
      (elixir 1.19.4) lib/enum.ex:4570: Enum.reverse/1                                                                                                                                          
      (xp_refactorex 0.1.52) lib/refactorex/refactor/dataflow.ex:21: XPRefactorex.Refactor.Dataflow.outer_variables/1                                                                           
      (xp_refactorex 0.1.52) lib/refactorex/refactor/function/extract_function.ex:42: XPRefactorex.Refactor.Function.ExtractFunction.refactor/2                                                 
      (xp_refactorex 0.1.52) lib/refactorex/refactor/function/extract_function.ex:2: XPRefactorex.Refactor.Function.ExtractFunction.visit/4                                                     
  Function: &:erlang.apply/2                                                                                                                                                                    
      Args: [#Function<0.114616880/1 in XPRefactorex.Refactor.available_refactorings/4>, [XPRefactorex.Refactor.Function.ExtractFunction]] 

I was able to replace it by calling outer_variables on any fragment which actually declares a variable - {:gen, _} is returned but never consumed. Swapping recursive_analyze to analyze_scope helped. Added a bunch of tests reproducing the issue.