Allow to set a default JDK per version #47
Description
Right now we have a single default version, which is used to determine the JDK to use when no specific version is requested. (It's also the JDK that ~/.jbang/currentjdk points to)
At some point we will likely have multiple JDKs with the same (major) version, for example when enabling external providers like "sdkman" and "scoop" or because we implemented Make it possible to have multiple JDKs installed of the same version. In those cases it would be nice to be able to select a default JDK per version.
One possible way of doing this might be to run jbang jdk version <major-version> <id-of-jdk> which would set the given JDK to be the default for that major version (of course the code would check that the versions match, but that's something we already do for jbang jdk install <major-version> <path-to-jdk>, so that' nothing new)