[1.21.2] Implement builtin mod resource/data pack sorting

[Apollounknowndev]

Supersedes #3913, targets 1.21.2 and changed how sorting works.

Overriding data/resource pack files from other mods is currently quite difficult as mod loading order isn't consistent. This can be especially annoying for compatibility mods that exist to override data/resource files from other mods.

This PR addresses the issue by implementing a system for mods to define in their fabric.mod.json what their priority is in the internal Fabric Mods data/resource pack relative to other mods. For example, a pack that overrides trees from a biome mod can set itself to load above the biome mod's datapack internally to make sure files are consistently overriden.

Mods that wish to change their priority relative to other in the load order can add this to their fabric.mod.json file:

"custom": {
    "fabric:resource_load_order": {
        "before": [
          "mod_a",
          "mod_c",
        ],
        "after": [
          "mod_d"
        ]
    }
}

Mods in the before list will load their resources before/under the mod, mods in the after list will load their resources after/above the mod.

[modmuss50]

I think this is fine, I havent looked at it in depth, but the idea seems solid. I think we discussed this before, but are we happy to have this in the fabric.mod.json as opposed to the pack.mcmeta file? I think im happy either way.

This clearly needs some tests, if you want some help setting up multiple testmods for one module let me know, it should be tottaly possible to do.

[Apollounknowndev]

It would be possible to move this to the pack.mcmeta file, but it'd require refactoring how mod packs are registered in an awkward way (Mod resource packs would need to have their profiles built, then sorted, then registered) and requires an additional accessor to get the resource pack metadata so I didn't opt for it. If there's an advantage to moving it to pack.mcmeta that makes it worth the hassle lmk.
I've tried to setup test mods but I'm not sure how to do the gradle wizardry, some help on that would be much appreciated.

[modmuss50]

I have added 3 blank test mods, hopefully this will help you make some tests out of them. I also made some small code cleanup/changes with unit testing in mind, but ran out of time. Feel free to take a look at this, if not I might come back to it.