This commit removes three unstable Deno APIs:
- "Deno.spawn()"
- "Deno.spawnSync()"
- "Deno.spawnChild()"
These APIs were replaced by a unified "Deno.Command" API.
Refactors the `Deno.Command` class to not handle any state, but only being an intermediary to calling its methods, and as such any methods and properties besides `output`, `outputSync` & `spawn` have been removed. Interracting with a `spawn`ed subprocess now works by using the methods and properties on the returned class of the `spawn` method.
