item: add a way to distinguish economic and non-economic stone materials

[myk002]

either with a new parameter or by augmenting the df.dfhack_material_category infrastructure

[quietust]

Is the intent to determine whether a stone is marked as economic (and would be excluded by most jobs) or whether the stone has potential economic uses (i.e. if the game would mark it as economic by default)?

[myk002]

I believe it's the latter definition that the requester (which I apparently forgot to cite) was going for