feat: add the ability to format more than one project at a time.

[jaudiger]

Add the ability to format more than one project at a time by authorizing the usage of multiple -p arguments.

Part of #75

[kylewlacy]

LGTM

I did find one thing that could be improved: if you run brioche fmt --check -p project1 -p project2 and both projects have formatting issues, then only the files for one of the projects will get printed

But I also don't feel that this is a blocker, I'm fine merging this as-is and handling that as a future issue or follow-up

[jaudiger]

Thank you, I'll do that in a follow-up. I always prefer to reduce the scope of my PRs. To reduce the review burden, the regression's scope, etc. At the end there are more PRs opened, but that's my trade-off