Upgrade Font Awesome to 5.15.1

[ggrossetie]

Introduce an attribute to configure the version (font-awesome-version).

I also found a workaround to support brand icons.
You can declare the following:

icon:rust[2x,role=fab]

And then, add the following styles in your custom stylesheet:

/* Font Awesome 5.x Brand compatibility */
span.icon.fab > i.fa {
  font-family: "Font Awesome 5 Brands";
  font-weight: initial;
}

resolves #363

[mojavelinux]

icon:rust[2x,role=fab]

This is likely out of scope for this change, but I at least want to bring up the point. I think we should standardize on set like is used by Asciidoctor PDF. See https://github.com/asciidoctor/asciidoctor-pdf#font-based-icons

[ggrossetie]

This is likely out of scope for this change, but I at least want to bring up the point

Yes, I was just experimenting 🧪
It's worth noting that I didn't introduce any change to the converter, I was showcasing a workaround that can be used today.

I think we should standardize on set like is used by Asciidoctor PDF. See asciidoctor/asciidoctor-pdf#font-based-icons

I was trying to address Olivier concern about introducing a new syntax: #304 (comment) but I also think that we should use the set attribute.
Anyway, I will create a dedicate issue and pull request to track this change.

[mojavelinux]

Super!

Btw, if you want to implement the workaround, you might consider defining both role and set and eventually dropping role once support for set is implemented. Just an idea.