Standardise on new quarto chunk style

[hadley]

Using knitr::convert_chunk_header() to convert (for next spring cleaning)

[hfrick]

Do we want type = "yaml" which is what the quarto docs use

 #| echo: false,
 #| fig.width: 10

or the (default) type = "multiline"?

#| echo = FALSE,
#| fig.width = 10

The help page for knitr::convert_chunk_header() says the yaml style isn't yet implemented but it looks like that is outdated info and it works nonetheless.

[jennybc]

FWIW when I do this conversion, I feel like I always switch to the type = "yaml" form.