Support for custom CSS

[dmi3kno]

Here's a great example for callout styling
Perhaps some of it could be parameterized from YAML, otherwise, some examples may be provided for styling of custom callouts

[coatless]

I'm aiming to avoid exposing the CSS/SCSS side as much as possible for a couple of reasons:

1. We want to leaning as heavily as possible on the built-in quarto.Callout node instead of a custom Div as other filters can now pick up the inserted callout.
2. As we expand to LaTeX, Typst, and other formats, there are going to need to be a cascading amount of effects.

That said, I have a quick note on styling in the FAQ:

https://quarto.thecoatlessprofessor.com/custom-callout/qcustom-callout-faq.html#what-if-i-want-to-change-the-appearance-of-all-my-custom-callouts

I think the better part of valor here would be to add another key like: enable-style: false. This would prevent the extension from running any of the generate*Format() portions in favor of allowing theme information to be loaded through:

format:
  html:
    theme: 
      light: [default, customstyle.scss]
      dark: [darkly, customstyledark.scss]

We'd also probably provide a general list of rules that need to be set via documentation.

Does that sound like a fair trade-off?