Pass the child to the ATL scope

[Gouvernathor]

transform lucy_jump_scare(child):
    child      # Show the original child.
    pause 5
    "lucy mad" # Jump scare.
    pause .2
    child      # Go back to the original child.

This is taken from the doc, about the special child parameter. However, there's no way of doing that cleanly in the show-with-atl statement:

v "But..."
show eileen vhappy:
    2.
    "eileen screamer"
    .1
    "eileen vhappy"
e "Hmm..."
show eileen sad

In this example, if the player clicks too fast, the change to sad is overwritten by the ATL to screamer then to vhappy.
What I propose is this:

show eileen vhappy:
    2.
    "eileen screamer"
    .1
    child

The child variable inside the ATL scope gets set to its child.
The possible interfering with the child parts of #4371 will need to be clarified, but in my view it should be done with a context.context.setdefault("child", child), so that this behavior kicks in only in cases when it would have raised an exception.