feat: support rdf:LangString datatype (+test)

[Yucukof]

As of current, datatype rdf:langString is not supported.

If we try to convert the following shex

<#AccessNeedDescriptionShape> {
  a [ interop:AccessNeedDescription ] ;
  interop:inAccessDescriptionSet  IRI  // shex:reference <#AccessDescriptionSetShape> ;
  interop:hasAccessNeed           IRI  // shex:reference <#AccessNeedShape> ;
  skos:prefLabel                  rdf:langString // <- Unknown datatype
}

Then the code generation fails because it applies the type as a raw string.

SyntaxError: ';' expected. (204:18)
      202 | export type ResourceShape = {
      203 | id: string; // the url of a node of this shape
    > 204 | description: http://www.w3.org/2000/01/rdf-schema#langString;
          |                  ^
      205 | size: number; // Size of Container
      206 | mtime: number; // Time of Container creation created
      207 | modified: Date; // Time the Container was modified

      185 |     // prettier formatting
      186 |     const prettierConfig = await prettier.resolveConfig(process.cwd());
    > 187 |     const formatted = prettier.format(content, {
          |                                ^
      188 |       ...prettierConfig,
      189 |       filepath: generates,
      190 |     });

      at e (node_modules/prettier/parser-typescript.js:1:411)
      at Object.parse (node_modules/prettier/parser-typescript.js:1:3072809)
      at Object.parse (node_modules/prettier/index.js:13625:19)
      at coreFormat (node_modules/prettier/index.js:14899:14)
      at format (node_modules/prettier/index.js:15131:14)
      at node_modules/prettier/index.js:57542:12
      at Object.format (node_modules/prettier/index.js:57562:12)
      at src/generate.ts:187:32
      at fulfilled (lib/generate.js:13:24)

The following code change fixes the issue without breaking the test.

[ludwigschub]

Other than that its good 👍

[Yucukof]

Sorry, this took me way more time than it should have...

[ludwigschub]

@Yucukof
I also thought about it a bit more and I am still questioning whether this is the right code to be generated. This also goes into #140 and eventually we'll need to implement the creation params in shex-methods.
Instead of:

} else if (valueExpr?.datatype === ns.rdf('langString')) {
    return toCreate ? 'string | Literal' : 'string';
...

We should force developers to supply the language with the string when they create shapes and similarly we should return the full literal node that includes the language when reading shapes; so more fitting would be:

} else if (valueExpr?.datatype === ns.rdf('langString')) {
    return 'Literal';

Sorry that I am only bringing this up now, but I didn't have much time to maintain this lib atm. I will make these changes to this PR and #140. Thank you for pointing this out 👍