Missing support for JEP 512

With Java 25 approaching, it would be nice to support [JEP 512](https://openjdk.org/jeps/512). I provided a pull request at https://github.com/mojohaus/exec-maven-plugin/pull/484. It supports the new forms of main (instance, non-public, no args), considered in the order laid out in the JEP. 