Add JDK 19 Virtual Thread preview support

I am using Conquerant in one of applications and am upgrading the entire application suite to JDK 19. I have decided to use JDK Virtual thread preview in a low impact application which is using Conquerant. While specifying custom Executor works, I have raised a PR to use Virtual threads by default if available. In case, Virtual threads are not available, it falls back to using regular platform threads

Feel free to suggest improvements :)