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

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

Priorities

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[donovanglover]

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