Enhance indexing speed with java vector api

[calixtus]

JabRef complains on startup with a warning:

WARN: Java vector incubator module is not readable. For optimal vector performance, pass '--add-modules jdk.incubator.vector' to enable Vector API.

The Vector API is a feature on hold (incubator) https://openjdk.org/jeps/460 since JDK 16 waiting for project Valhalla https://openjdk.org/projects/valhalla/ to be finished eventually and be finally integrated. It should already be stable and there should be no problem for us to enable that feature.