Add a new `Timer` class that operates in seconds (using `TimeType`) alongside the existing `TickTimer` which uses `TickType`. Rename the original `Timer` to `TickTimer` throughout the codebase. Update `ClientWorld` to maintain separate maps for tick-based and time-based timers, and replace the hardcoded footstep timer in `ClientPlayer` with a time-based callback timer. Also add `play_2d` to `AudioEngine` and load an ambient bird sound to demonstrate the new timer functionality.
