Please implement `Process#toHandle`

Right now, `Process` methods delegating to it (like `Process#descendants`) fail with UOE:
```
java.lang.UnsupportedOperationException: class com.pty4j.unix.UnixPtyProcess.toHandle() not supported
  at java.base/java.lang.Process.toHandle(Process.java:737)
  at java.base/java.lang.Process.descendants(Process.java:808)
  at ...
```