Add a --check flag to the formatter CLI (#6982)
## Summary
Returns an exit code of 1 if any files would be reformatted:
```
ruff on charlie/format-check:main [$?⇡] is 📦 v0.0.286 via 🐍 v3.11.2 via 🦀 v1.72.0
❯ cargo run -p ruff_cli -- format foo.py --check
Compiling ruff_cli v0.0.286 (/Users/crmarsh/workspace/ruff/crates/ruff_cli)
Finished dev [unoptimized + debuginfo] target(s) in 1.69s
Running `target/debug/ruff format foo.py --check`
warning: `ruff format` is a work-in-progress, subject to change at any time, and intended only for experimentation.
1 file would be reformatted
ruff on charlie/format-check:main [$?⇡] is 📦 v0.0.286 via 🐍 v3.11.2 via 🦀 v1.72.0 took 2s
❯ echo $?
1
```
Closes #6966.
