Implement slot-specific attribute modifiers

[calcastor]

In ProjectAres, it was possible to specify a slot in which an attribute is applied to an item, as exemplified below (an excerpt from the global include for Overcast Network):

<item-mods>
    <rule>
        <match>
            <material>diamond leggings</material>
        </match>
        <modify show-attributes="false">
            <attribute amount="6" slot="slot.armor.legs">generic.armor</attribute>
            <attribute amount="400" slot="slot.armor.legs">generic.armorToughness</attribute>
        </modify>
    </rule>
</item-mods>

In this example, this option is helpful because it avoids applying these attributes when the item is held, only applying it if worn.