ability to format imported files

```
==> justfile <==
import 'sub.just'

main_var := 'variable'

@_:
just --list

@main:
echo main
echo {{ main_var }}

==> sub.just <==
@sub:
echo sub
echo {{ main_var }}
```

`just --fmt --unstable` will format only `justfile`

`just --fmt --unstable -f sub.just` raises an error

```
error: Variable `main_var` not defined
——▶ sub.just:4:10
│
4 │ echo {{ main_var }}
│ ^^^^^^^^
```

I see to ways to resolve this:

- `--fmt` should format imported files automatically
- `--fmt` can omit undefined variables and just format syntax (doesn't seem right)

Reproduce the issue with the shown justfile and sub.just files using `just --fmt --unstable` and `just --fmt --unstable -f sub.just`. Then inspect the formatter and imported-file handling behind these CLI entry points. Done means the intended formatting behavior for imported files is implemented and the undefined `main_var` failure no longer blocks the relevant formatting workflow.

Written by the indexing model from the issue text.