Improve generated vocab performance

[berezovskyi]

The Python script to generate RDF vocab classes should be updated not to generate QName object instances all the time but instead use a lazy-init backing field.

eg instead of

public static QName ActedOnBehalfOf => QNameFor("actedOnBehalfOf");

to use

private static QName? _actedOnBehalfOf;
public static QName ActedOnBehalfOf => _actedOnBehalfOf ??= QNameFor("actedOnBehalfOf");

[berezovskyi]

Since C# 14, we can just do

public static QName ActedOnBehalfOf => field ??= QNameFor("actedOnBehalfOf");

[berezovskyi]

Also, we can use a ConcurrentDictionary like in eclipse-lyo/lyo.designer#303 to also extend performance improvements to other props under a namespace generated on the fly. Although we should add a switch or similar to prevent adding field-backed QNames to the dictionary for no reason.