If `format: revealjs` and there are `webr` cells on a slide, slide should be `scrollable` by default

[mine-cetinkaya-rundel]

It's hard to predict how much space the output of a webr cell will be, so I'm finding myself adding the {.scrollable} class to every slide with a webr cell. And, of course, forgetting to do so every once in a while. It would be nice if the existence of a webr cell made the slide automatically scrollable. There may be reasons to not do this (either technical or not), but so far in my use-case, I can see it would be helpful.

[georgestagg]

Great idea, thanks! EDIT: Implemented in 926fb88.