StandardSystemProperty: Support jdk.module.* properties

[cpovirk]

See docs. They "may" be included in a given system. I'm not sure if that's intended to be a different state from java.vendor.version ("optional") or java.compiler (which is just missing in practice). And of course they didn't exist at all until Java 9.

We haven't heard demand for these -- and I see few usages in some quick searches, aside from jdk.module.path in jOOQ -- but maybe we'll want to someday. For example, it might come up if we extend ClassPath to support the module path (internal issue 172826019). (That said, we've heard good things about ClassGraph as an alternative/replacement.)

The properties are:

• jdk.module.path
• jdk.module.upgrade.path
• jdk.module.main
• jdk.module.main.class

If we do add them, we'll want to update the Javadoc I'm about to submit that says we do not include them :)

[cpovirk]

Oh, and there's also java.vendor.version (added in Java 11, listed as optional as of Java 13). It also looks rarely used.