Core/Spells: move spell resistance calculation into separate function (and some cosmetic changes)

Ref #7254

src/server/game/Entities/Unit/StatSystem.cpp

@@ -184,6 +184,12 @@ bool Player::UpdateAllStats()
     return true;
 }
 
+void Player::ApplySpellPenetrationBonus(int32 amount, bool apply)
+{
+    ApplyModInt32Value(PLAYER_FIELD_MOD_TARGET_RESISTANCE, -amount, apply);
+    m_spellPenetrationItemMod += apply ? amount : -amount;
+}
+
 void Player::UpdateResistances(uint32 school)
 {
     if (school > SPELL_SCHOOL_NORMAL)