Feature: Background task support

[guybedford]

Long-running tasks like deno --watch or servers can be useful to depend on to ensure they are initialized along with the task being run.

It could be useful to support a background = true option for tasks, where the task is invoked, but completion is not waited on before trrigering the parent task that depends on it.

Further, it can be useful to have the ability to automatically restart the task when it fails or to have background tasks that get restarted when their dependencies change. Perhaps background = 'watched' or background = 'keepalive' could be useful ways to achieve these.