Refactor special_step_sounds out of existence, Human steps now depend on leg type, Digitigrade legs use claw footstep sfx (#87006)

## About The Pull Request

1. Deletes `special_step_sounds`. Unused anyways. 
2. Human footstep type is now determined by its legs. 
- This means a human with two different legs, with two different
footstep types, now alternate between footstep type every other step.
3. Digitigrade legs now use claw footstep sfx

## Why It's Good For The Game

Less random species vars, more dynamic human code, and a little bit more
immersion

## Changelog

🆑 Melbert
refactor: Refactored footsteps for humans. Human footstep sound effects
are now determined by your leg type. Report any oddities.
qol: Digitigrade legs now play claw footstep SFX. "plat plat" is dead,
long live "tap tap".
/🆑

code/modules/unit_tests/focus_only_tests.dm

@@ -50,3 +50,6 @@
 
 /// Ensures only whitelisted planes can have TOPDOWN_LAYERing, and vis versa
 /datum/unit_test/focus_only/topdown_filtering
+
+/// Catches any invalid footstep types set for humans
+/datum/unit_test/focus_only/humanstep_validity