Standardize Mapping content identifiers

[nichtich]

See https://github.com/gbv/jskos-tools#mapping-identifiers. At least mapping content identifier should be optional part of the standard (starts with urn:jskos:mapping:content:). Implementation

• https://github.com/gbv/jskos-tools/blob/f04c3a857af0ded92174f75fe6931ce1c62a9301/lib/identifiers.js#L63-L64
• https://github.com/gbv/jskos-tools/blob/f04c3a857af0ded92174f75fe6931ce1c62a9301/lib/identifiers.js#L26-L46