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 (#4990)
  - work formatting when there is newline or comment between argument and comma.

    ```crystal
    def foo(a
            ,)
    end

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

    is formatted to correctly:

    ```crystal
    def foo(a)
    end

    def foo(a # comment
            )
    end
    ```
  - format arguments with 2 spaces indentation (#1924)
  - distinguish comment followed on newline and argument.
    The formatter keeps this for example:

    ```crystal
    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!!

* Rename 'wrote_newline' inside 'format_def_arg' to 'just_wrote_newline'

And fix some conditions for readbility.