Add map difficulty limit

First, I apologize to all players who tried the game during the ​​Steam Next Fest​. Due to my oversight in lacking difficulty guidance for levels, some players entered later-stage maps with high difficulty without any equipment or attribute enhancements. This resulted in a poor gaming experience for you. I sincerely apologize again.
Difficulty restrictions have now been added. Only after clearing Dream Forest on Nightmare difficulty will the normal difficulty of the subsequent 4 levels be unlocked.
Even so, I still recommend playing in this order: Dream Forest → Dark Wilderness → Destroyed RedRidge → Frost Forest → ShadowSwamp → Magma Hell. Monster health and damage in later maps are extremely high. Only by enhancing attributes or equipping legendary gear during mid-game can you withstand the challenges of later maps.
Once again, I apologize to all players who tried the game during the ​​Steam Next Fest​.