Support background command by sys-arg "%delay"

From this feature, any command can be run in background by adding a sys-level arg %delay:

Another example, we use %delay in a flow named delay-demo:

What delayed-commands look like in flow desc (aka, command -):

When we run delay-demo, the executing panel shows how and when those delayed-commands are scheduled:

The is a bg area for background running info (zZ is scheduled but not started yet, >> is running, ✓ is started and finished):

When main thread is end, each backround command's output will be showed one by one:

Important: the env will be a snapshot at the scheduling time of main thread, any changes in the snapshot(a copy) will not affect the original env in main thread.