Add support for # %% for chunk demarcation in knitr::spin

[kylebutts]

Hi,

The R VSCode extension has nice support for inline code chunks for R scripts using the # %% comment to separate code chunks. This small change would integrate with this and allow a seamless .R notebook script to report pipeline

This will also help with quarto-dev/quarto-cli#6660 allowing to render .R scripts with quarto
https://quarto.org/docs/prerelease/1.4/script.html

Running knitr::spin("test.R", format = ".qmd) on this

#'---
#'title: "test"
#'---

# %% setup
#| message = FALSE,
#| warning = FALSE
library(tidyverse)

v = seq(-pi, pi, 0.01)
data = data.frame(
  x = v, y = sin(v)
)

# %% 
ggplot() + 
  geom_point(
    aes(x = x, y = y),
    data = data
  )

produces a valid qmd document:

---
title: "test"
---

```{r setup}
#| message = FALSE,
#| warning = FALSE
library(tidyverse)

v = seq(-pi, pi, 0.01)
data = data.frame(
  x = v, y = sin(v)
)

```
```{r}
ggplot() + 
  geom_point(
    aes(x = x, y = y),
    data = data
  )
```

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[yihui]

Looks great! Could you add a news bullet to https://github.com/yihui/knitr/blob/master/NEWS.md#new-features? Then I'll merge this PR. Thanks!