[1.21.3] Allow mods to add custom FeatureFlags

[XFactHD]

This PR makes it possible for mods to add custom FeatureFlags to the vanilla feature flag system. Since the FeatureFlagRegistry holding the flags is set in stone long before mods start loading, the flags are specified in a JSON file instead of being collected by an event or similar system. To allow adding more than the 64 flags allowed by vanilla (a limitation that is not unlikely to be hit with 100s of mods in a pack), the flag masks are extended by a long[] in the FeatureFlagSet and an offset field in FeatureFlag for indexing into the array.

Closes #1106.

This PR is the 1.21.3 port of #1538.

[neoforged-pr-publishing]

• Publish PR to GitHub Packages

Last commit published: 49d3b0aa7dce9cb08bf3eaf8ac8b3478191697a1.

PR Publishing

The artifacts published by this PR:

• 📦 net.neoforged:testframework:21.3.2-beta-pr-1619-modded_feature_flags_21.3
• 📦 net.neoforged:neoforge:21.3.2-beta-pr-1619-modded_feature_flags_21.3

Repository Declaration

In order to use the artifacts published by the PR, add the following repository to your buildscript:

repositories {
    maven {
        name 'Maven for PR #1619' // https://github.com/neoforged/NeoForge/pull/1619
        url 'https://prmaven.neoforged.net/NeoForge/pr1619'
        content {
            includeModule('net.neoforged', 'testframework')
            includeModule('net.neoforged', 'neoforge')
        }
    }
}

MDK installation

In order to setup a MDK using the latest PR version, run the following commands in a terminal.
The script works on both *nix and Windows as long as you have the JDK bin folder on the path.
The script will clone the MDK in a folder named NeoForge-pr1619.
On Powershell you will need to remove the -L flag from the curl invocation.

mkdir NeoForge-pr1619
cd NeoForge-pr1619
curl -L https://prmaven.neoforged.net/NeoForge/pr1619/net/neoforged/neoforge/21.3.2-beta-pr-1619-modded_feature_flags_21.3/mdk-pr1619.zip -o mdk.zip
jar xf mdk.zip
rm mdk.zip || del mdk.zip

To test a production environment, you can download the installer from here.

[ApexModder]

Working locally in mod dev via pr-publishing 👍

_{Example is custom item depending on a custom experimental feature flag, level has said flag disabled}

[ApexModder]

Update IMenuTypeExtension.create to allow modders to flag their MenuTypes if desired.
_{Reposted from Discord}