make organize imports more flexible about import placement

[Techatrix]

The current implementation of source.organizeImports (#2051) always places the imports at the top of the file. Sometimes you find code that has all imports at the bottom of the file or some imports between code. It would be nice if ZLS could this.

I propose either of these two solutions:

• add a special case where ZLS will place all imports at the bottom of the file (no config option)
• never move imports to a different location, only sort imports that are next to each other i.e. not separated by an empty line