[data-shapes] Add a node expression type for calling a node expression (#279)

bergos has just created a new issue for https://github.com/w3c/data-shapes:

== Add a node expression type for calling a node expression ==
It was previously [mentioned](https://github.com/w3c/data-shapes/issues/222#issuecomment-2643206166) that node expressions cannot use an IRI as an identifier because they would be interpreted as a constant term. To workaround that problem, a new node expression type that accepts the IRI of a node expression as the key parameter should be defined.

@afs proposed `sh:call`:
```turtle
 [ sh:call my:nodeExpr_numTables ]
```

Since there doesn't seem to be a better place for it, we should consider making this a node expression type defined in the core specification.

Please view or discuss this issue at https://github.com/w3c/data-shapes/issues/279 using your GitHub account


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