Optimizes /proc/icon_exists() (#89357)

## About The Pull Request
This PR reimplements https://github.com/tgstation/tgstation/pull/71538
atop `master`. Quoting the original PR:

> Every `icon_exists()` call will cache the entire file. Past me didn't
realise _why_ file opts were so expensive, but I do now. This is
immeasurably slower on a single call, and _significantly_ faster on
subsequent calls to the same file.

I attempted to handle some of the review comments that were posted
there, by splitting screaming functionality into its own proc.

* `if(icon_state in icon_states(file))` and `if(!(icon_state in
icon_states(file)))` were refactored to use `icon_exists(file,
icon_state)`.
* Where screaming was seemingly wanted (and where there wasn't a more
descriptive error inside the `if` block), I refactored them to use
`icon_exists_or_scream(file, icon_state)`
* The exception to the above was under
`/datum/unit_test/turf_icons/Run()` and
`/datum/unit_test/worn_icons/Run()`, where `icon_states()` was being
passed a mode flag. Given that this is only used in unit tests (where
performance isn't a priority), I opted to leave these be.

Additionally, I revised the documentation comment for
`/proc/icon_exists()`, as I felt it was a bit vague currently.

## Why It's Good For The Game
https://youtu.be/Z9G1Mf6TZRs

## Changelog
No player-facing changes (hopefully).

---------

Co-authored-by: SyncIt21 <110812394+SyncIt21@users.noreply.github.com>

code/modules/unit_tests/suit_storage_icons.dm

@@ -35,12 +35,12 @@
 			continue
 
 		if(worn_icon) //easiest to check since we override everything.
-			if(!(icon_state in icon_states(worn_icon)))
+			if(!icon_exists(worn_icon, icon_state))
 				log_test("\t[count] - [item_path] using invalid [worn_icon_state ? "worn_icon_state" : "icon_state"], \"[icon_state]\" in worn_icon override file, '[worn_icon]'")
 				count++
 			continue
 
-		if(!(icon_state in icon_states('icons/mob/clothing/belt_mirror.dmi')))
+		if(!icon_exists('icons/mob/clothing/belt_mirror.dmi', icon_state))
 			already_warned_icons += icon_state
 			log_test("\t[count] - [item_path] using invalid [worn_icon_state ? "worn_icon_state" : "icon_state"], \"[icon_state]\"")
 			count++

code/modules/unit_tests/turf_icons.dm

@@ -9,7 +9,7 @@
 		var/icon_file = initial(turf_path.icon)
 		if(isnull(icon_state) || isnull(icon_file))
 			continue
-		if(!(icon_state in icon_states(icon_file)))
+		if(!icon_exists(icon_file, icon_state))
 			TEST_FAIL("[turf_path] using invalid icon_state - \"[icon_state]\" in icon file, '[icon_file]")
 
 	for(var/turf/closed/mineral/turf_path as anything in typesof(/turf/closed/mineral)) //minerals use a special (read: snowflake) MAP_SWITCH definition that changes their icon based on if we're just compiling or if we're actually PLAYING the game.
@@ -18,7 +18,7 @@
 		var/icon_file = initial(turf_path.icon)
 		if(isnull(icon_state) || isnull(icon_file))
 			continue
-		if(!(icon_state in icon_states(icon_file)))
+		if(!icon_exists(icon_file, icon_state))
 			if(modular_mineral_turf_file && (icon_state in icon_states(modular_mineral_turf_file, 1)))
 				continue
 			if(!(icon_state in icon_states('icons/turf/mining.dmi', 1)))
@@ -45,13 +45,13 @@
 
 		var/list/burnt_states = instanced_turf.burnt_states()
 		for(var/state in burnt_states)
-			if(!(state in icon_states(damaged_dmi)))
+			if(!icon_exists(damaged_dmi, state))
 				TEST_FAIL("[open_turf_path] has an invalid icon in burnt_states - \"[state]\", in '[damaged_dmi]'")
 
 
 		var/list/broken_states = instanced_turf.broken_states()
 		for(var/state in broken_states)
-			if(!(state in icon_states(damaged_dmi)))
+			if(!icon_exists(damaged_dmi, state))
 				TEST_FAIL("[open_turf_path] has an invalid icon in broken_states - \"[state]\", in '[damaged_dmi]'")
 
 	run_loc_floor_bottom_left = run_loc_floor_bottom_left.ChangeTurf(initial_turf_type) //cleanup.