Weaken Wither Structure Requirements

[WaitingIdly]

changes in this PR:

• made is so the wither pattern applies '~', unused -> true instead of BlockWorldState.hasState(BlockMaterialMatcher.forMaterial(Material.AIR)), meaning any blocks will match it, not just AIR.
• made is so forming the wither will not replace the side bottom blocks.
• tied to a single config option (default: false)

[what-the-diff]

PR Summary

• Introduction of Weaker Wither Structure Requirements
  This new feature gives users more flexibility when creating 'Withers' by allowing non-air blocks to be used in the bottom corners of a structure. This results in a convenient user experience.

• Adjustments to UTConfigTweaks.java
  This file has been updated with a new boolean variable utWitherPlacement. This variable is specifically programmed to support our new feature of weakening Wither structure requirements, essentially making it an on/off switch for this feature in our codebase.

• Addition of UTWitherFormationMixin.java
  This new file has the mixin for modifying the rules of how the wither is formed. This change makes our new feature - Weaken Wither Structure Requirements, functional.

• Inclusion of mixins.tweaks.blocks.witherstructure.json
  To perfectly sync our wither structure changes, we added this new configuration file. It enables us to properly configure the desired changes in Wither structure according to our requirements. Without this configuration, implementation of our new feature would've been problematic.