Add `Pid()` to get the processId associated

[terrywh]

I'm building a tool which needs to track the started cpty process. try adding a function that provide the pid of it.

[UserExistsError]

Thanks for the PR. Can we return an int instead? The stdlib exec package does this so might be good for consistency:

https://cs.opensource.google/go/go/+/refs/tags/go1.22.2:src/os/exec_posix.go;l=72

[terrywh]

converted to int.