Need a shield API

[rikka0w0]

Description

There was FabricShieldLib, but it was not updated for 1.21.2 and above. I think it is time to have this available in Fabric API, as this is a commonly used function.

The vanilla code uses xxx instanceof ShieldItem and itemStack.isOf(Items.SHIELD) to check if an item should be considered as a shield. If someone wants to implement his/her own shield, he/she would need to mixin these two points with @ModifyConstant and @reDIrect. Without doing this, the modded shield won't be able to block damage. These two mixins can only be applied once and will prevent others from adding more shields. One possible walkaround is to use the chainable @ModifyExpression from MixinExtra, like this:

https://github.com/rikka0w0/Fabric-Shield-Lib/blob/de00cea01f8a4aef2dede903ac988ed147d6a54f/src/main/java/com/github/crimsondawn45/fabricshieldlib/mixin/LivingEntityMixin.java#L83
https://github.com/rikka0w0/Fabric-Shield-Lib/blob/de00cea01f8a4aef2dede903ac988ed147d6a54f/src/main/java/com/github/crimsondawn45/fabricshieldlib/mixin/PlayerEntityMixin.java#L54

However, we cannot guarantee everyone follows the same standard. Moreover, the injection point may change as the Minecraft codebase evolves.

Hence, we need to make the shield API available in Fabric API. It does two things:

1. Allows modded items to be considered as shield
2. Allows modded shields to have different cooldown times when disabled by axe

In addition, modders will have a standard way of checking whether an item should be considered a shield.

In the implementation of the shield API, we can treat everything in tag ConventionalItemTags.SHIELD_TOOLS (Or another new tag, namely fabric:shield) as shield:

	public static boolean isShieldItem(Item item) {
		return item.getRegistryEntry().isIn(ConventionalItemTags.SHIELD_TOOLS);
	}

Then, we use MixinExtra's @ModifyExpression to change the behavior of xxx instanceof ShieldItem and itemStack.isOf(Items.SHIELD).
Modders can simply include their modded shields in ConventionalItemTags.SHIELD_TOOLS with data generators, and the modded shields will be able to block damage.

Sometimes, a modder may want to set a different cooling downtime for the modded shield. To achieve this, the shield API can introduce a new DataComponentType COOLDOWN_TICKS, a non-negative integer. If a shield item has this DataComponent, the cooldown tick will be the value set in the DataComponent. Otherwise, use vanilla cooldown ticks (100).

I have an implementation here:
https://github.com/rikka0w0/Fabric-Shield-Lib/tree/experimental

If everyone is happy, I can start to work on a PR to bring this feature to Fabric API.