Add mixins for FabricItem.getAttributeModifiers (1.21)

[opekope2]

Added an ItemStack mixin to call FabricItem.getAttributeModifiers(ItemStack).
Similar to #3797, but for 1.21.
Resolves #3787

I tested it on a dev client&server and a prod client&server, and I didn't encounter any issue.

[modmuss50]

I will merge this in 1.21 now, and get the 1.20.6 PR out another time 👍

[modmuss50]

This fix is broken (#3812) FabricItem.getAttributeModifiers should be tottaly removed as you can now use the component. I will revert and deprecate for 1.20.6, and remove fully in 1.21

[opekope2]

This API allowed Items to override attribute modifiers based on certain conditions of the ItemStack. Here's an example where I override it, which delegates to AnvilHandler and other classes. The alternative I see is changing the attribute modifier where I modify the ItemStack, but that only gets called when a player presses a key, and not when /give is used, so initially it would not be functional as intended, only after the player presses the key. NeoForge supports this using IItemExtension.getAttributeModifiers(ItemStack) as of 1.20.6, but I don't know about their future.

[modmuss50]

Unfortunately I'm struggling to follow the Kotlin code sorry, you can set the default item componet in the Item settings, this would then apply to /give.

The vanilla none stack aware method is deprecated, and ingored if the stack has a attribute modifier component.

[opekope2]

My use case is to change the attribute modifiers based on my custom attribute modifier. For /give, I'll try Item.postProcessComponents(ItemStack)

[opekope2]

The removal makes sense if I look at mojmap