[FEAT] add file complexity

## Description

The output with the `-a` or `-t` flag collects information about both file length and number of commits. Both could be combined to calculate the complexity or importance of a file. A smaller file that is changed often might be more important to a project than a large file that is only changed once per year.

## Technical Details

- [ ] add a function in `impl File` in `file.rs` that calculates the complexity
- [ ] add the complexity to the tables in the `grouped_info` function
- [ ] possibly also add an option to only display the complexity (needs a new flag in the clap command, and a new display function)