Allow in thread response in `respondFn`

[baileywickham]

app.action provides a respond function for convenience. The respond function allows you to respond in channel, update the previous message, or respond with an ephemeral message. It would be nice if there was an option to respond in the thread in which the last message was sent to allow for threaded conversation without needing to use the client directly. Let me know if this seems reasonable and I can take a look at implementing it!