Switch to parsing with {parsermd} instead of regex

[coatless]

Promising new entrant that more intelligent reads through the structure of an RMarkdown/Markdown document.

c.f. rundel/parsermd#17

[coatless]

Suggest changing over to new Knitr engine features in v1.37:

https://yihui.org/en/2022/01/knitr-news/

verbatim and embed look the most promising.

````{verbatim}
We can output arbitrary content verbatim.

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

The content can contain inline code like
`r pi * 5^2`, too.
````

```{embed, file="foo.R"}
```

Only issue is the new way of specifying code chunk options introduced in v1.35

R style:

```{r}
#| echo = FALSE,
#| fig.width = 10
```

YAML style:

```{r}
#| echo: false
#| fig.width: 10
```