Use `java_home` on Mac for detecting installed JDKs

See https://github.com/jbangdev/jbang/issues/1385#issuecomment-1166705563

java_home on Mac actually has the possibility to list all installed JDKs and give the path to selected versions. It might be very useful to be able to use that information when dealing with //JAVA and --java options. We could avoid having to install JDKs if they are already available on the system. (You'd still be able to install JDKs using jbang jdk install and those JDKs would have priority over the system ones).

If we do this we might want to consider extending this somehow to other OSes that don't have a java_home command. Either by hard-coding in support for a bunch of well-known package managers (probably a lot of work/maintenance) or by allowing some kind of plug-in system where making a java_home-like script/command available would enable the same kind of functionality (but where do those plugins come from and how would the user install them? Could we perhaps use Jbang itself, writing those plugins as Jbang scripts?)