Add gradle workflow support for Eclipse IDE

Off the top of my head I've tried using the eclipse plugin (this should prepare "static" classpath entries pointing at local gradle caches). It almost works... almost because we have references between sub-atomic project elements (tests and main) that make Eclipse see these as circular (because Eclipse treats project sources and main classes as one).

I pushed this code to jira/LUCENE-9438. Perhaps there are ways of making it work. I'm not a big fan of having a single "blob" project with all the sources and classpaths combined (the IDE won't help you figure out what's accessible from a given subproject then) but maybe it's the only way to make it work for Eclipse, don't know.