Proposed changes to `Deno.spawn()` API

[nayeemrmn]

• SpawnOutput should extend ChildStatus instead of composing it, discussed favourably on Discord a while ago.
• SpawnOutput::stdout, SpawnOutput::stderr, Child::stdin, Child::stdout and Child::stderr shouldn't be optional, instead make 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), so we should make them always present.