fn:parse-json, fn:json-to-xml: number-parser, fallback

[ChristianGruen]

1. See JSON Parsing & Serialization: Numbers #33: number-parser option needs to be added to fn:json-to-xml.
2. Similar to fn:replace, I would suggest using function(xs:untypedAtomic) as item()?) as signature for both the fallback and the number-parser option, and (for fallback) to invoke fn:string#1 on the result. This way, explicit casts in the code get obsolete. Queries like the following one…

parse-json(
  '-123',
  map { 'number-parser': fn($n) { $n => number() => abs() }
)

…can then be simplified to parse-json('-123', abs#1).