Change AccordionInputBinding.receiveMessage to async

[wch]

This fixes #872. It should be used with rstudio/shiny#3930.

With this PR, this what the test app from #872 looks like when it starts:

[wch]

Note that I wasn't able to successfully re-run yarn because I have a different system version of yarn from what others are using. (Once #788 is merged then we'll be able to consistently run yarn without worrying about the system version.)

Before releasing, we'll want to update the shiny version referred to in DESCRIPTION and in package.json.

[wch]

Just to add a bit of detail: each message sent (via the R functions accordion_panel_insert() and accordion_panel_open() ends up putting a message on the JS side in the actionQueue, where they are handled as serially -- by awaiting each message handler.

This PR makes receiveMessage async, and within it, it awaits each call to renderContent. These changes (along with the PR in Shiny) ensure that the messages are handled in order, without any interleaved processing of them.