HTML screenshot in PDF with webshot2 requires PNG format

This was reported in knitr

• https://github.com/yihui/knitr/issues/2276

where a warning has been added.

• check that it is thrown in Quarto

Screenshot for HTML widgets needs to be done with png format, and not pdf, because webshot2 with PDF and Chrome is not good enough

This will work

---
title: test
format: pdf
---

```{r}
#| label: fig-ash-map
#| fig-cap: "Map of Town of Ashland"
#| fig-format: png 

library(sf)
library(leaflet)

ashland <- st_read('https://services2.arcgis.com/sKZWgJlU6SekCzQV/arcgis/rest/services/Ashland_Boundary/FeatureServer/0/query?outFields=*&where=1%3D1&f=geojson', quiet = TRUE)

leaflet(ashland) %>% 
  addTiles() %>% 
  addPolygons()
```

format: pdf in Quarto will default to fig-format: pdf, which means the Screenshot will be not good by default

Maybe should somehow make it the default, here or in knitr. cc @yihui

Later improvement in webshot2 R package may help with this also.

Originally posted by @cderv in https://github.com/quarto-dev/quarto-cli/issues/3279#issuecomment-1824040068