Changes

Delete the generic 1 field loaders

Replaced by RecordLoadable and the various related loadables

Make mapped loadable map record loadables into record loadables

Implement compact loadable

This allows a record loadable to read its value from a primitive, and conditionally serialize to a primitive.
It technically would work for a list type as well and you can use the class for that, but did not add helpers for it (as there is no list loadable interface and compact can be deceptive on primitives)

Provide a special nullable NBT field for NBTLoadable

The packet buffer methods using NBT already are nullable, we made them non-null in NBTLoadable. However, if you want a nullable field, using them natively is more compact

Cleanup implementations of fluid stack and item stack loadables

Instead of writing a full loadable from scratch, it uses record loadable builder methods (except for item stacks with NBT, which I had to special case due to a dumb forge API)
Takes advantage of the new record mapping, and the new compact loadable.
In addition, both item and fluid stacks support fixed size variants, which will ignore count in JSON in favor of a fixed size (1 for items, 1000 for fluids, though a method exists to let you choose other sizes)

Rework default field a bit

Loadable#field is now Loadable#requiredField to make it clear its a required field, will be going through usages and making sure all want required.
No longer override getAndDeserialize (now called getIfPresent) as that messes with some of the loadable builders (and notably cannot be built int). Plus, you might want to requrie a field that has its own default
Instead, going to encourage creation of custom defaultField impls without a default parameter for such usages
Loadable also now has a getOrDefault helper, same semantics as getIfPresent. Simplifies some field implementions

Improve ItemOutput serialization and deserialization

Now takes full advantage of ItemStackLoadable to allow 2 loading options:
* Optional output is now handled directly in the API, so you can have an ItemOutput that resolves to empty, or disallow that entirely. Note tag output may still resolve to empty due to an empty tag.
* Fixed count is now an option, as some cases using ItemOutput don't support stack sizes greater than 1 (e.g. casting recipes).

In addition, ItemOutput.EMPTY now exists for those wanting an empty instance, and the default serialize function takes a boolean for whether to write count.
Left the old networking logic in place for now, it will be less compact as it always writes count and writes a boolean for empty (instead of just using items.AIR for empty). However, its easier to invoke so figured I'd leave it until more stuff migrates to loadables

Remove unused GSON instance in AddEntryLootModifier

Fix missing return keyword

Add a method to ItemOutput to support tag outputs

Create DefaultLoaderRegisty for the defualt instance logic

Lets me make all the stuff related to default not nullable/throw as I can guarantee its present by instance

Fix mistake in parsing element predicates in fallback predicate registry