Feat: added shape tree discrimination through Resource class

[Giacomo-Papaluca]

As I explained in this issue and this issue, I think that a little addition to shape tree semantics is needed in order to fully distinguish this shape tree

<#TaskTree>
  a st:ShapeTree ;
  st:expectsType st:Resource ;
  st:shape pm-shex:TaskShape .

from this one

<#ProjectTree>
  a st:ShapeTree ;
  st:expectsType st:Resource ;
  st:shape pm-shex:ProjectShape ;
  st:references [
    st:hasShapeTree <#TaskTree> ;
    st:viaPredicate pm:hasTask
  ] .