Turn the "Run Shiny App" button into "Stop" once it's running

[nealrichardson]

It would be nice for a clean way to stop running the app. Currently I have to go to the terminal where it is running and hit ctrl-C, but that still leaves the panel open with the app in it, so I then have to close that. Maybe the "Play" triangle button could turn into a "Stop" square button?

[jcheng5]

This is so bare-bones right now because we are re-using the VS Code Terminal and Simple Browser features, which AFAIK are fire-and-forget. Simple Browser especially was supposed to be a stopgap, we really ought to create our own browser that we totally control; JJ did this for the Quarto extension and suggested I copy the code from there, but I never got to it.