Fix CaveGenMixin not preserving vanilla behavior

[inboundbark]

Enabling the cave generation toggle made caves spawn differently from vanilla even without changing values from their vanilla defaults. It also didn't replicate r1.6 caves when changing the size to 40 and rarity to 15.

[what-the-diff]

PR Summary

• Refined Code Modification
  Switched from utilizing the ModifyArg to ModifyExpressionValue in the import statement. This step offers a more accurate way of modifying the code.

• Cave Generation Size Update
  The utCaveGenSize function, that determines the size of caves, has been improved. Previously, it was taking variable values but now, it has been modified to use fixed values for size generation, which makes the output more consistent.

• Cave Generation Rarity Upgrade
  Similar changes have been made to utCaveGenRarity, a method that governs the rarity of cave generation. This function has also been updated to use a set value, also known as a 'constant', rather than a variable.

• Enhanced Clarity and Maintainability
  These changes to the two methods increase the clarity of our code and make it easier to maintain over time. Utilizing constants for value injection instead of method calls simplifies the understanding of what each piece of code does.