hey i was just wondering if its possible to add wither drops?

this seems to have worked, this also means that it does not always drop the star the value can change this way it seems balanced

so i added to the commonconfig in your code
builder.push("Wither Drop Settings");
WitherDropStar = builder.comment("Enable/Disable Wither drops.")
.define("WitherDropsStar", false);
builder.pop();

added this to Lootmanager
// Wither Support.
if (Boolean.TRUE.equals(filteredLootDrops.isEmpty() && COMMON.WitherDropStar.get())
&& mobType.equals(BossMonster.WITHER) && random.nextInt(9) == 0){

  filteredLootDrops.add(new ItemStack(Items.NETHER_STAR));
}

Thanks for the feedback and the example. I you want, you could create a PR (Pull Request) for this.
Otherwise I'm happy to implement this in the next update.

not good at doing the pull requests