jn:parse-json needs options and be able to handle sequences

[Matthias Brantner]

The jn:parse-json function needs to be adapted according to the JSONiq spec.

Quote spec:

This function has the same semantics as fn:parse-xml(), except that it parses the string as JSON (not XML), and returns an Object or Array rather than an XML document.
jn:parse-json($arg as xs:string?, $options as object()) as json-item()*
jn:parse-json($arg as xs:string?) as json-item()?
If $options is specified, the EBV of $options("jsoniq-multiple-top-level-items") is computed. If it is true, then jn:parse-json recognizes and parses sequences of objects and arrays, with no separation other than whitespaces.