Add support for the `allShortestPath` and `shortestPath` constructs

[marijnvanwezel]

The allShortestPath and shortestPath constructs are currently not supported. Note that these are not functions, but rather a special kind "path":

Invalid input 'RETURN': expected "(", "allShortestPaths" or "shortestPath" (line 1, column 7 (offset: 6))
"MATCH RETURN n;"