add unit method SetImmuneTo

[55Honey]

Adds a new method to add and remove mechanic immunities. See azerothcore#167

Tested on AC with:

local function ImmunityOff(eventid, delay, repeats, creature)
    creature:SetImmuneTo(26, false)
    creature:SendUnitSay("I am interruptable now.", 0)
end

local function Cast(eventid, delay, repeats, creature)
    creature:CastSpell(creature, 57775, false)
end

local function EnterCombat(event, creature, target)
    creature:SetImmuneTo(26, true)
    creature:SendUnitSay("You can not interrupt me.", 0)
    
    creature:RegisterEvent(Cast, 5500, 0)
    creature:RegisterEvent(ImmunityOff, 15000, 1)
end

local function LeaveCombat(event, creature)
    creature:RemoveEvents()
end

RegisterCreatureEvent(113, 1, EnterCombat)
RegisterCreatureEvent(113, 2, LeaveCombat) -- OnLeaveCombat
RegisterCreatureEvent(113, 4, LeaveCombat) -- OnDied

[55Honey]

I do not mind adjusting the PR, but I have no idea what it fails the build for. It compiles just fine on AC with the same 2 changes. If anyone has an idea, feel free to advice.