`just --fmt`: allow passing files to format

Common linter integrations like [treefmt](https://github.com/numtide/treefmt) pass the files that should be formatted as arguments to a formatter. Would be great if just could support this for easy integration with such tools.

Start at the `just --fmt` command and trace how formatter arguments are parsed and passed through. Determine how file arguments should interact with the existing formatting behavior, then verify that named files are accepted and formatted correctly for integrations such as treefmt.

Written by the indexing model from the issue text.