Ghasts Spawn only at low light levels. Ghast Farms broken.

Between 1.14 and 1.16, Ghasts were changed internally from being a "Mob" to being a "Monster". By default, monsters only spawn when the light level is low. (This is hardcoded in the game and cannot be changed by json-based spawning rules).

Expected Behavior: Ghasts spawn at all light levels.

Actual Behavior: Ghasts only spawn if the light level is below 8.

Consequence: Portal-based ghast farms no longer work because portals emit light.

Demo world: Press a button to switch from a netherwart block floor to a shroomlight floor.

How to fix: Add a Ghast::isDarkEnoughToSpawn() function that always returns true. Example code below. This wasn't needed before when Ghasts were Mobs, but now is needed because they are Monsters.

virtual bool Ghast::isDarkEnoughToSpawn() const override {
     return true;
}