Drop trailing comma

[danwwilson]

I'm using breaker of chains in a {targets} pipeline and had the experience of the following code

 my_target = some_function(p1, p2) |>
    do_thing(),

> Error in parse(text = broken_chain) : <text>:2:22: unexpected ','

I understand this could create other challenges, but, it would be great if the {breakerofchains} was able to drop the trailing , to work nicely in targets pipelines. I'm not sure if that creates issues because it's in a list() or tar_plan() object or not though.