Support JEP 425: Virtual Threads (JDK 19)

[testforstephen]

This issue is used to track the work to support "Virtual Threads" in JDK 19.

https://openjdk.java.net/jeps/425
https://jdk.java.net/loom/

[AlanBateman]

JEP 425 integrated into JDK 19 in early May for build 22.
Links to the JDK 19 schedule, downloads, and docs can be found here:
https://jdk.java.net/19/