feat: add flag to jump to next empty workspace

[plsmphnx]

Description of changes

This change adds an --empty flag, which causes hyprnome to advance to the next empty workspace as if it were on the last workspace (or first, when combined with --previous). It's conceptually equivalent to Hyprland's "empty" workspace target, with the hyprnome benefits of skipping intermediate empty workspaces and staying on the same monitor.

Things done

• Formatted code with cargo fmt
• Checked for linting issues with cargo clippy
• Tested that the binary works as expected with cargo build
• Ran software tests with cargo test
• Commits follow Conventional Commits

Priorities

Add a 👍 reaction to pull requests you find important.

[donovanglover]

Thanks for the PR! Definitely a useful feature to have and apologies that I haven't gotten around to reviewing yet.