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The help page for knitr::convert_chunk_header() says the yaml style isn't yet implemented but it looks like that is outdated info and it works nonetheless.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

[jennybc]

The 2025 tidy upkeep bullets now recommend knitr::convert_chunk_header(type = "yaml"). Arguably we can close this, because I'm not sure we really need to do anything more in usethis. I.e. the function that does the conversion appears to already exist in knitr. The only possible upgrade would be for usethis to detect the presence of older, in-header chunk options (instead of nudging unconditionally).