Color standardization, vars moved, and signals (#52574)

Defined all the existing light_color values.
    Moved their definitions to colors.dm
    Made white the default color. It was so already, but that was very obscured.
    Moved the atom light-related variables to the atom definition.
    Wrapped changes to variables such as light_color into procs that report the event through signals.
    Moved the light_on variable to the atom level, also adding a signal for its changing, to represent toggling lights.
    Cleaned up a little bit of code in where new variables were defined before redefinitions.

This is all atomization to reduce changes in #52413
None of this affect gameplay at all, it's all code cleaning and refactoring.

There's more colors to standardize, a search for color = will find lots of targets, and I see little need to have both the LIGHT_COLOR and COLOR patterns, but I don't want to make this PR bigger than it already is.

code/modules/modular_computers/computers/item/computer_ui.dm

@@ -160,7 +160,7 @@
 			return 1
 
 		if("PC_toggle_light")
-			light_on = !light_on
+			set_light_on(!light_on)
 			if(light_on)
 				set_light(comp_light_luminosity, 1, comp_light_color)
 			else
@@ -178,7 +178,7 @@
 					to_chat(user, "<span class='warning'>That color is too dark! Choose a lighter one.</span>")
 					new_color = null
 			comp_light_color = new_color
-			light_color = new_color
+			set_light_color(new_color)
 			update_light()
 			return TRUE