[data-shapes] Add support for Node Expressions targets (#339)

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

== Add support for Node Expressions targets ==
It should be possible to use Node Expressions to compute targets. I'm not sure yet if we should support it for all targets we currently have or only for `sh:targetNode`. The others would be just shortcuts for additional triple pattern matching, which can also be done in Node Expressions. SPARQL target would no longer be required, as it can be computed with a Node Expression.



**Allow Node Expressions for all targets:**
- pro:
  - All allowed target parameters would be consistent
  - The first level is easier to analyze
- con:
  - Adds a bit of complexity to the implementations

**Allow Node Expression only for `sh:targetNode`**
- pro:
  - Easier to implement
  - A single place for the definition of dynamic targets
- con:
  - Allowed target parameters would be inconsistent
  - Node Expressions can get a bit more complex

Please view or discuss this issue at https://github.com/w3c/data-shapes/issues/339 using your GitHub account


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