core: move expansion checks into module header

mods/equip-compare.lua

@@ -1,12 +1,9 @@
 local _G = _G or getfenv(0)
-local GetExpansion = ShaguTweaks.GetExpansion
-
--- already part of the game
-if GetExpansion() ~= "vanilla" then return end
 
 local module = ShaguTweaks:register({
   title = "Equip Compare",
   description = "Shows currently equipped items on tooltips while the shift key is pressed.",
+  expansions = { ["vanilla"] = true, ["tbc"] = nil },
   enabled = true,
 })