Reordering granularity

I like when reordering feature is applied to variables inside the file.

But I'd prefer to avoid reordering the functions.
- Sometimes I have a public function and a related private helper. It reads better when this private function located right after use. When code reordering is enabled, it might end up after a bunch of public functions.
- Sometimes I have static private helpers and they come first in the file. I read code top to bottom, so I'd prefer them to be located after when they used by public or virtual functions.

I know it contradicts the official style guide, but it would be nice to have a CLI option that enables only variables reordering.

Start by tracing the formatter's existing reordering option and its handling of variables and functions. Define completion as a CLI mode that reorders variables while leaving functions in source order, with tests covering both behaviors.

Written by the indexing model from the issue text.