[feat] Allow setting `lx fmt` backend in `lux.toml`

Unless I'm mistaken, the formatting backend can only be specified on the command-line via the `--backend` flag. Being able to set this in the `lux.toml` would be very helpful, possibly under a `[format.backend]` key. This table could also include an `args` key to pass additional args to the backend, and `include` and `exclude` keys with patterns of files to include or exclude.