Explore/Add ability to inject code into a custom code cell

[coatless]

This would be similar to {{< include >}} shortcode, e.g. {{< include _content.qmd >}}; but maybe work inside of a markdown structure?

This isn't quite a situation where I want the official {quarto-ext/include-code-files} extension as that would require:

```{.r include="script.r"}
```

Instead, I'm looking for something more actionable for custom code cells.

```{shinylive-r}
#| standalone: true
#| viewerHeight: 600

{{<embed-code file="path/to/shiny/app/" >}}

```

This is to tide over the request in quarto-ext/shinylive#37