Urn constructor
Constructor.
As per RFC 8141 Section 3.1, the NSS and NID are converted to lower-case.
Implementation
Urn({
required String nid,
required String nss,
this.fragment = '',
this.rComponent = '',
Map<String, String>? qComponents,
}) : nid = nid.toLowerCase(),
nss = nss.toLowerCase(),
_qComponents = Map.from(qComponents ?? {});