handle new prefix in fontawesome 5

[cderv]

This will fix #1554

Following fontawesome update to V5 in #1340, there was some missing changes for Rmarkdown website, for which icon can be set in navbar using icon:

With this change, I believe it will not break current behavior as

• Ionicons can still be used (we have a very old version but it is still working) and class will be ion ion-...
• Fontawesome icons can be passed as before fa-home. in that case fas prefix will be added - the new default one.
• Fontawesome icons can also be passed as full name like fab fa-r-project. In that case, the full name will be used as class for the span.

I have chosen to not be specific on the prefix using fa\\w, but we could only check for supported prefix fa(b|s|r|l|d) instead.

This is a quick fix without rethinking the all logic for icons.

I did not find any documentation in the package.

Maybe I should update https://bookdown.org/yihui/rmarkdown/rmarkdown-site.html ?

[apreshill]

This is great! I wonder if this help page could be made to be more helpful? https://rmarkdown.rstudio.com/docs/reference/html-dependencies.html

[cderv]

Oh yeah we could add information on the versions of each dependency, specifically external ones like these icon libs. Thanks for pointing this doc.

[yihui]

Looks good. Thanks!