Implement player death attribute persistence tweak

[Invadermonky]

Player attributes are reset on player death. This causes changes to base attribute values, such as those modified with the Player Attribute Commands mod, to return to their default values.

Attribute modifiers, such as those applied through potions, are not copied during this process.

By default, this tweak does not cause any attributes to be persistent and will require setup by any users wishing to utilize this feature.

[what-the-diff]

PR Summary

• Improved user documentation
  The README.md file was updated to explain Attribute Keeper, a new feature that allows certain attributes of a player to continue even after the player "dies" in the game.

• Enhanced core functionality

  • In UniversalTweaks.java, a necessary step for this new feature was handled - importing and merging it with the game's foundation code.

  • In UTConfigTweaks.java, an option has been added under name 'utAttributeLock' that enables users to specify which attributes of a player should persist post their death. Useful instructions, examples accompanying the same have been added too, assisting users to master it quicker.

• Introduced new class for attribute management
  A new file, UTAttributeKeeper.java was introduced. This is a pivotal part for the new attribute management as it ensures that base player attributes, as specified by the user, continue to exist even post the player's death.