Roundtripping with `:::` div and code cell removes necessary blank line

In https://github.com/posit-dev/positron/issues/11205 we have a report of some problems round-tripping between visual and source mode.

If you have .qmd syntax like this, with the needed blank lines around the code cell inside the callout box:

---
title: "My simple Quarto file"
format: html
---

## Make a plot

You can embed plots in a Quarto file, for example:

::: callout-tip
## Tip with Title

This is an example of a callout with a title.

```{r}
#| echo: false
plot(faithful)
```

:::

## Cool math

Or you can do simple calculations:

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

Then switch to visual mode, then switch back to source mode, you lose the blank line after the code cell, to get this:

---
title: "My simple Quarto file"
format: html
---

## Make a plot

You can embed plots in a Quarto file, for example:

::: callout-tip
## Tip with Title

This is an example of a callout with a title.

```{r}
#| echo: false
plot(faithful)
```
:::

## Cool math

Or you can do simple calculations:

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

This is a pretty significant problem, because now we run into the parsing problems when you don't have those kind of blank lines around code cells such as outlined in https://github.com/quarto-dev/quarto/issues/493.

Reproduce issue 884 with the provided .qmd example by switching from source mode to visual mode and back. Compare the resulting callout and code-cell boundaries with the original, and review the related reports in posit-dev/positron#11205 and quarto-dev/quarto#493. Done means the blank line after the code cell is preserved and the resulting document avoids the reported parsing problems.