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/admin/view_variables/view_variables.dm

@@ -54,7 +54,7 @@ ADMIN_VERB_AND_CONTEXT_MENU(debug_variables, R_NONE, "View Variables", "View the
 			else // it means: icon_state=""
 				if(!dmi_nullstate_checklist[icon_filename_text])
 					dmi_nullstate_checklist[icon_filename_text] = ICON_STATE_CHECKED
-					if("" in icon_states(image_object.icon))
+					if(icon_exists(image_object.icon, ""))
 						// this dmi has nullstate. We'll allow "icon_state=null" to show image.
 						dmi_nullstate_checklist[icon_filename_text] = ICON_STATE_NULL