Simplify to vector when parsing json message for R

[cderv]

So that we get vector instead of unnamed list as e.g some knitr options expect vector. This closes #3909

Context

Setting multiple for dev was not working

---
title: "Document"
format: 
  html:
    embed-resources: false
knitr:
  opts_chunk:
    dev:
      - png
      - pdf
      - svg
---

```{r}
library(ggplot2)
ggplot() + ggtitle("this is a title")
```

because Quarto parses the YAML then pass it to the R process using JSON message that is parsed with jsonlite but knitr$opts_chunk$dev is seen as a unamed list and not coerced to a vector.

This creates issue within knitr which expect a vector for some options and currently does not handle or coerce list to vector itself.

I believe that YAML parsing does correctly see this as a vector, so it seems ok to simplify when parsing the JSON.
This will also convert to Matrix and Dataframe in some case - if we want to prevent that we could do

request <- jsonlite::parse_json(input, simplifyVector = TRUE, simplifyDataFrame = FALSE, simplifyMatrix = FALSE)

but seems too much.

If this is too broad fix anyway, We could simplify only the knitr fields in the message.;

BTW I tried to add a test using the new smoke-all folder - Hopefully adding a file is enough for it to be run.

[jjallaire]

It seems unlikely that a data frame or matrix would ever be passed so I agree that's too much. Will merge as is. Thanks!