Incorporate more webr in exercises

[jayhesselberth]

It's currently a little clunky, but a future release is going to be great:

https://r-wasm.github.io/quarto-live/

In particular, the need for {{< include ../../_knitr.md >}} on every page is brittle; it would be nice if it was activated for an entire site, instead of per-page.

[nmukherjee]

hold for now

think later about incorporating into exercises