Fix #1924
Close #4980, #4990

This is just refactoring but it contains many bug fixes and improvements. These come from refactoring effects, so I cannot separate them from this.

Bug fixes and improvements:

• work formatting double splat with trailing comma (Correctly work formatting double splat argument with trailing comma #4990)

• work formatting when there is newline or comment between argument and comma.

  def foo(a
          ,)
  end
  
  def foo(a # comment
          ,)
  end

  is formatted to correctly:

  def foo(a)
  end
  
  def foo(a # comment
          )
  end

• format arguments with 2 spaces indentation (Formatter indentation not matching with what I'd expect #1924)

• distinguish comment followed on newline and argument. The formatter keeps this for example:

  def foo(a # argument 'a' is ...
          )
  end
  
  def foo(a
          # more argument here in future...
          )
  end

And, perhaps there are other bug fixes and improvements because format_def_args has too many bugs to remember.

Refactoring is so great!!

I apologize to @asterite and other members for big commit. But I'm sure this is the best solution because fundamental problem is format_def_args is so complex. If we resolved issues explained in above separately, then format_def_args would becomes more complex and it means refactoring becomes harder. I don't hope such a thing.

Please read the source code. I believe it is clearer and simpler then past.

Thank you.