[core] Support property types "File", "ClassLoader"

[adangel]

[core] Support property types "File", "ClassLoader"

[oowekyala]

I would suggest using Path instead of File for consistency.

Java defines the auxClasspath - while the TODO says, it should be a ClassLoader, I disagree. It should be a path list thing so that we are not tied against java.lang.ClassLoader (which is closeable, ...). Let's call it "classpath", maybe need to define an own type with own validations.

I think the idea here was to be able to provide an arbitrary implementation. Maybe the more abstract Classpath type from the java module is more appropriate. I imagined this would be useful in eg IDE integrations, which may use weird classpath tricks. But I haven't researched this and am not sure there is a use case out there. Probably it would be enough to do as you say and just use a List or something. We could use just a String for simplicity, but ideally we would avoid using a platform specific path delimiter.