Formal type value for index defintions of the new luceneNg indices

[bhabegger]

Currently OAK is using a very old version of lucene (4.7) for it's indices of type "lucene". There have been different attempts in upgrading lucene to newer versions but this have not succeeded and with time this has become more complicated. Recently (see apache#2817) a new approach to solving the issue was not to upgrade lucene but rather reimplement a new type of index directly based on a more recent version of lucene (9+) with in mind to make the code base easier to upgrade (also noting that since 4.7 the lucene SPIs have stabilized).

Now it is time to give this new index type a formal type property value. Currently the the code in the PR is using lucene9 but this is not ideal. Why ? Because one of the goals is to be able to "upgrade" lucene even major versions so this would mean have a type=lucene9 index used by lucene 10 code... luceneNg (new generation) could be an option, but then once the legacy lucene if fully deprecated then there will only be luceneNg. Another idea could be to mark the type with the year it was introduced (independently of which version it first used). This would give lucene2026 or (lucene26 but this could give the false impression that it is associated with 26 version of lucene).  As this the kind of thing that could stick for a long time, I think it's worth some thought before going head down into using it. So what do. you think ?