[knitr] Correctly write markdown content to rmd intermediate on windows

[cderv]

On Windows markdown content parsed by Quarto from .qmd file is passed to R part of code base as a single string using windows EOL (\r\n). Previously, using base::write() was writing an empty line between each original linebreak because it is based on cat() and does write linebreak for \r and \n. We need the rmd intermediate as close as possible to the input .qmd

Opening this PR to run tests but I believe this will be ok and this change is for the best.

Example

Take this simple .qmd

---
title: "Test"
format: html
---

## Quarto

Quarto enables you to weave together content and executable code into a finished document. To learn more about Quarto see <https://quarto.org>.


```{r}
1 + 1
```

What Quarto is passing to R is this markdown content

markdown <-
"---\r\ntitle: \"Test\"\r\nformat: html\r\n---\r\n\n\r\n## Quarto\r\n\r\nQuarto enables you to weave together content and executable code into a finished document. To learn more about Quarto see <https://quarto.org>.\r\n\r\n\r\n\n```{r}\r\n1 + 1\r\n```\r\n"

Note the \r\n.

This would be the content of .rmarkdown intermediate used by rmarkdown::render() - see all the new lines.

---

title: "Test"

format: html

---




## Quarto



Quarto enables you to weave together content and executable code into a finished document. To learn more about Quarto see <https://quarto.org>.






```{r}

1 + 1

```

I am surprised that this is has not create any issue, probably because we only knit() using rmarkdown but still.

This PR fix this on the R side by making sure we write markdown the right way in the .rmarkdown input file.

We could also normalize markdown to have only \n and no \r\n EOL.