DMN16 — Add an itemKind property to ItemDefinition

SDMN, BPMN, and CMMN have a property that specifies the kind of element that is being modeled. The set of enumerations for this property include: data, conceptual, physical, etc.
SDMN is directly referencing DMN’s ItemDefinition, but is extending the element to include this “itemKind” property.
If this property makes sense in the DMN context and is added in DMN 1.4, then SDMN would not have to extend ItemDefinition for this property.
SDMN extends ItemDefinition in other ways, which will be addressed by separate issues.