- "SpawnOutput" extends "ChildStatus" instead of composing it
- "SpawnOutput::stdout", "SpawnOutput::stderr", "Child::stdin",
"Child::stdout" and "Child::stderr" are no longer optional, instead
made them getters that throw at runtime if that stream wasn't set
to "piped".
- Remove the complicated "<T extends SpawnOptions = SpawnOptions>"
which we currently need to give proper type hints for the availability of
these fields. Their typings for these would get increasingly complex
if it became dependent on more options (e.g. "SpawnOptions::pty"
which if set should make the stdio streams unavailable)
