AFAIK Lua has no constants.
MMExtension has const.* - https://grayface.github.io/mm/ext/ref/#Constants , don't forget to look there.
SPELL_SUMMON_WISP_ID format is debatable. Something like const.Spell.SummonWisp might be better (iteration through namespace, yes). And there might already be table somewhere in Game or evt or elsewhere.
Also, spell id of specific spell could be changed even during game progress, if I understand it correctly.
Memory addresses on the other hand are strong candidates for const.*. const.MemAddr maybe. So for this example something like const.MemAddr.SpellBeingCasted somewhere like Scripts/General/1_0memaddr.lua
Also, there is Scripts/Core/offsets.lua
Ah. Didn't know that. Yeah, let's go with yours, that nomenclature scheme for constants is what we have always used at work, so I didn't even think about it being different for this project / lua / etc. :^)