extend the domain and range of `skills`

[VladimirAlexiev]

Currently skills applies only to JobPosting, Occupation. IMHO it should also apply to Person, Organziation:

• To fill a posting, the Person must have the requisite skills: so it should be possible to record this against the person
• Similar for an org: it may possess collective skills being the sum of its employee's skills, or its own "emergent" skills

[github-actions]

This issue is being nudged due to inactivity.

[MatthiasWiesmann]

This sounds straightforward, any objection?

[philbarker]

Should be OK so long as the definition gets tweaked.

[alex-jansen]

Fixed in #3651