isURI, isBlank and isLiteral are missing in LQML

[jerdeb]

Add the isURI function in LQML (like typeof), which accepts a ?s or ?o parameter and checks if it is a valid URI or not.

The isLiteral function accepts ?o and checks if the object is a literal. We can expand this to other datatype literals in the future

The isBlank function accepts ?s ?o and checks if the subject or the object are blank nodes

[clange]

@jerdeb you may also want to consider the two-argument variant isLiteral(?o, datatype) (e.g. isLiteral(?o, xsd:date)); see my comment in the JDIQ paper. - @ahsaniqbal please take note