Adds a slithering/crawling noise to footstep selection (#5551)

## About The Pull Request
title

## Why It's Good For The Game
more consistent snake noises or whatever if you lack feet

## Proof Of Testing
if it compiles it works. it's a one line addition
<img width="774" height="98" alt="image"
src="https://github.com/user-attachments/assets/a742ed21-fda1-4d16-88af-51769e982bc3"
/>

https://discord.com/channels/1059199070016655462/1061433713088536706/1502705781181644810
<details>
<summary>Screenshots/Videos</summary>

</details>

## Changelog

🆑
add: Crawling/slithering noise to footstep noise selection, for those
that lack legs or those feeling Invisible.
/🆑

modular_zubbers/code/modules/client/preferences/footstep_sound.dm

@@ -4,7 +4,7 @@
 	savefile_key = "footstep_sound"
 
 /datum/preference/choiced/footstep_sound/init_possible_values()
-	return list("Default", "Shoes", "Highheels", "Claws", "Hooves")
+	return list("Default", "Shoes", "Slither", "Highheels", "Claws", "Hooves")
 
 /datum/preference/choiced/footstep_sound/create_default_value()
 	return "Default"
@@ -21,6 +21,8 @@
 	var/static/list/value_to_define = list(
 		"Shoes" = FOOTSTEP_MOB_SHOE,
 		"Claws" = FOOTSTEP_MOB_CLAW,
+		"Slither" = list(list(
+			'sound/effects/footstep/crawl1.ogg'), 40, 1),
 		"Highheels" = list(list(
 			'modular_zubbers/sound/effects/footstep/highheel1.ogg',
 			'modular_zubbers/sound/effects/footstep/highheel2.ogg',