Refactor a bunch of old APIs, add some new APIs, and add fallback addon

[dfsek]

This does a whole bunch of stuff, at this point it's basically "dfsek's original 7.0 ideas lite"
Let me know if there are any specific things you want me to revert.

So far we have:

• Add functional-style API, and update a whole bunch of things to use that instead of exceptions or manually unwrapping data structures.
• Replace usage of Java's Optional with Maybe to play nice with new APIs.
• Refactor config shortcut stuff to allow applying shortcuts to any types, not just registry-backed types.
• Started work on an addon using said shortcut API to implement "blockstate fallbacks", a config type which will allow falling back to other blockstates if one is missing (due to updated Minecraft version changing IDs, a mod not being installed, etc). This will make it a lot easier to make config packs with mods in mind, and perhaps we could even use it in Overworld to support some popular mods. (Closes [Feature] Allow graceful and dynamic handling of block IDs. #475)

Planned:

• Rename addons to consistent scheme:
  • manifest-addon-loader -> addon-loader-manifest
  • api-addon-loader -> addon-loader-api
  • palette-block-shortcut -> shortcut-palette-block

There will probably be more things that I may or may not remember to add to this comment :)

[dfsek]

I have no idea why the CI is failing, I looked briefly and it was something about javadocs, I will investigate this later

[dfsek]

As I go i am also refactoring any getters I encounter to record-style getters. (thing() instead of getThing())