river-status: add keyboard_layout event

[MaxVerevkin]

I'm new to zig and server-side wayland, so please let me know if if there are better ways to do certain things.

sway was used as a reference.

• Simple test client: https://github.com/MaxVerevkin/river-kbd-layout-watcher
• Status bar integration: https://github.com/MaxVerevkin/i3status-rust/tree/river-kbd