fa spinner and play button icons don't work when self-contained: true

[alexCardazzi]

The spinner and play button of the rendered version of this code fails to load.

---
title: webR in Quarto HTML Documents
format: html
engine: knitr
filters:
  - webr
self-contained: true
---

This is a webr-enabled code cell in a Quarto HTML document.

```{webr-r}
print("hello")
```

[coatless]

So, the self-contained or embed-resources is not possible as we can't embed the webr client within the document itself.

You can see some prior discussions at #62.

[coatless]

With #168, you can use embed-resources if you are not using the service-worker communication channel.