Missing: Double-fenced language code blocks

[cscheid]

We technically don't need this anymore, but we should probably keep it around for a while because knitr depends on stuff like it:

```{{r}}
#| echo: true
#| output-location: slide
library(ggplot2)
ggplot(airquality, aes(Temp, Ozone)) + 
  geom_point() + 
  geom_smooth(method = "loess")
```

[cscheid]

The knitr semantics for this is that when knitr seems a language block with more than one curly brace pair, it removes one pair of braces and returns the code block unchanged.

We should just just the language block untouched, of course.