Wraps lowertext() to ensure proper stringification. (#82442)

## About The Pull Request

Fixes #82440

This PR just creates a new macro, `LOWER_TEXT()` (yes the irony is not
lost on me) to wrap around all calls of `lowertext()` and ensure that
whatever we input into that proc will be stringified using the `"[]"`
(or `tostring()` for the nerds) operator. very simple.

I also added a linter to enforce this (and prevent all forms of
regression) because I think that machines should do the menial work and
we shouldn't expect maintainers to remember this, let me know if you
disagree. if there is a time when it should be opted out for some
reason, the linter does respect it if you wrap your input with the
`UNLINT()` function.

code/modules/instruments/songs/play_legacy.dm

@@ -9,7 +9,7 @@
 	var/list/octaves = list(3, 3, 3, 3, 3, 3, 3)
 	var/list/accents = list("n", "n", "n", "n", "n", "n", "n")
 	for(var/line in lines)
-		var/list/chords = splittext(lowertext(line), ",")
+		var/list/chords = splittext(LOWER_TEXT(line), ",")
 		for(var/chord in chords)
 			var/list/compiled_chord = list()
 			var/tempodiv = 1

code/modules/instruments/songs/play_synthesized.dm

@@ -9,7 +9,7 @@
 	var/list/octaves = list(3, 3, 3, 3, 3, 3, 3)
 	var/list/accents = list("n", "n", "n", "n", "n", "n", "n")
 	for(var/line in lines)
-		var/list/chords = splittext(lowertext(line), ",")
+		var/list/chords = splittext(LOWER_TEXT(line), ",")
 		for(var/chord in chords)
 			var/list/compiled_chord = list()
 			var/tempodiv = 1