Implement texture handling for BlockItemBuilder texture overrides

[tiduslaugh]

Description

BlockItemBuilders ignore texture overrides provided via the .texture method, preventing scripters from changing a block item's texture without a workaround. See https://discord.com/channels/303440391124942858/1098748984572518440 . This fix attempts to mimic the behavior of ItemBuilder which handles this data correctly, but its generateAssetJsons method is overridden by BlockItemBuilder.

Example Script

// kubejs/startup/example.js
StartupEvents.registry('block', event => {
    event.create('dirt_pile')
        .model("kubejs:block/dirt_pile")
        .item(builder => {
            builder
                .displayName("Pile o' Dirt")
                // This workaround works because the modelJson attribute is processed in `generateItemModelJson`
                // .modelJson({
                //     parent: 'minecraft:item/generated',
                //     textures: {
                //       layer0: 'kubejs:item/dirt_pile',
                //     },
                // })
                // ...but this doesn't, because the textureJson attribute is not, and this item will have the parent model texture
                .texture("kubejs:item/dirt_pile")
        })
})

Other details

I'm very new to this codebase and MC modding in general so there is probably something I've overlooked :)