feat: support xsd:language datatype (+test)

[Yucukof]

This PR is similar to #139 and fixes the same issue with a different data type.

[ludwigschub]

@Yucukof Hm I looked around shex.io a little bit but I couldn't find the documentation regarding xsd:language nor could I run validation on an example:

PREFIX interop: <https://interop.example.com#>
PREFIX skos: <https://skos.example.com#>
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>

start = @<#AccessNeedDescriptionShape>

<#AccessNeedDescriptionShape> {
  a [ interop:AccessNeedDescription ] ;
  interop:inAccessDescriptionSet  IRI ;
  interop:hasAccessNeed           IRI ;
  skos:prefLabel                  xsd:language ;
}

PREFIX : <https://example.com#>
PREFIX interop: <https://interop.example.com#>
PREFIX skos: <https://skos.example.com#>

<AccessNeedDescriptionShape> 
  a interop:AccessNeedDescription ;
  interop:inAccessDescriptionSet  :Description ;
  interop:hasAccessNeed           :AccessNeed ;
  skos:prefLabel                  "EN" .

validating https://rawgit.com/shexSpec/shex.js/main/packages/shex-webapp/doc/AccessNeedDescriptionShape as https://rawgit.com/shexSpec/shex.js/main/packages/shex-webapp/doc/shex-simple.html#AccessNeedDescriptionShape:
    validating "EN":
      NodeConstraintError: expected to match <http://www.w3.org/2001/XMLSchema#language>
  OR
  Missing property: https://skos.example.com#prefLabel

Am I misunderstanding how it should be used? Also do you have an example in which shex is validating the input as xsd:language datatype?

[Yucukof]

I agree, it's kind of the weirdest format...
It takes the following shape.

interop:usesLanguage "en"^^xsd:language

Source: Data Interop Panel