[luxon] Tighten allowed unit values
[luxon] Tighten allowed unit values

georgemarshall/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,markogresak/DefinitelyTyped,magny/DefinitelyTyped,mcliment/DefinitelyTyped,magny/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,dsebastien/DefinitelyTyped,borisyankov/DefinitelyTyped