[Feature] Add a rule for spells to bypass stacking rules (#4716)

* define rule that allows for spells which always stack

* Update spells.cpp

---------

Co-authored-by: Akkadius <akkadius1@gmail.com>

zone/spells.cpp

@@ -3135,6 +3135,17 @@ int Mob::CheckStackConflict(uint16 spellid1, int caster_level1, uint16 spellid2,
 		}
 	}
 
+	const std::string& always_stack_spells = RuleS(Spells, AlwaysStackSpells);
+	if (spellid1 != spellid2 && !always_stack_spells.empty()) {
+		const auto& v = Strings::Split(always_stack_spells, ",");
+		if (Strings::Contains(v, std::to_string(spellid1))) {
+			return 0;
+		}
+		if (Strings::Contains(v, std::to_string(spellid2))) {
+			return 0;
+		}
+	}
+
 	/*
 	One of these is a bard song and one isn't and they're both beneficial so they should stack.
 	*/