Add support for niri workspaces

[vilhalmer]

I've been using this project in niri for a couple months now and it works great. I haven't written any Rust in a decade, so this may not be optimal, but I was able to put together workspace support based on the Hyprland provider. I'm not 100% sure I got every edge case, but I've been running it for 24 hours of normal use now without any crashes and it seems to behave across multiple outputs.

I chose not to add a dependency on the niri-ipc crate that was recently published because so little of it was actually needed. The socket helper within it also wasn't useful since it blocks. Additionally, the crate has a warning about not following the usual stability guarantees. Instead, I used a recent feature of serde to ignore any unknown event types so that any additions to the ipc protocol shouldn't result in errors.

niri has a concept of named workspaces, but they are the exception rather than the norm so I chose to append the name to the workspace number if it is set. I didn't make this format configurable since there's currently no setting to do so and defining a syntax for that seems like a can of worms. This seems like a good followup change, and I'm happy to change the format here in the meantime to whatever seems best.

I am very open to adjusting any and all of this since I have no idea what I'm doing :)

[vilhalmer]

I found one bug with multiple displays, will push a fix for that along with rustfmt as soon as it's solved.

Edit: done

[MaxVerevkin]

Thanks, looks good to me!

[vilhalmer]

Thanks!