update (core): Knock Back Helpers (#12639)

Adds knock back helpers to the core for future core and module use.

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

@@ -18727,6 +18727,7 @@ void Unit::KnockbackFrom(float x, float y, float speedXY, float speedZ)
             player->SetCanFly(true, true);
 
         sScriptMgr->AnticheatSetSkipOnePacketForASH(player, true);
+        player->SetCanKnockback(true);
     }
 }