Run the command and render its output.
Resolves with ExecOutput, or with whatever an onComplete callback returned in its
place. T defaults to ExecOutput so the common case needs no cast at the call site.
Optionalcommand: stringAlias for abort()
Optionalsignal: stringLog the final output. Can be overridden to customize output handling. Set to a no-op function to suppress output logging.
Optionaloutput: ExecOutputAttach to an already-started child and render its output.
Resolves with the command's ExecOutput, unless an onComplete callback returned a
value - that value is passed through instead. T names what the caller expects back;
it defaults to ExecOutput, which is what a run without an onComplete produces.
Abort/kill the running command