Use fontawesome R package for HTML dependencies

[cderv]

Use the fontawesome R package https://github.com/rstudio/fontawesome and deprecated the fontawesome dependency. https://github.com/rstudio/rmarkdown/blob/fedcdcf112971c5900c210c79aa08216b54808c3/R/html_dependencies.R#L185-L195

Some other package may use this dependency

• pagedown https://github.com/rstudio/pagedown/blob/db0764afa05c8b38f70bbfc1972bbb0ee3dffd43/R/resume.R#L17
• flexdashboard https://github.com/rstudio/flexdashboard/blob/33458eaae7c4164ed300a02be30bcb61d14a4f20/R/dependencies.RL4

and a lot of others: https://github.com/search?q=html_dependency_font_awesome&type=Code as this usage is quite widespread

htmltools::tagList(rmarkdown::html_dependency_font_awesome())

This should solves https://github.com/rstudio/rmarkdown/issues/2042 and https://github.com/rstudio/rmarkdown/issues/1661 by relying on the other package to update the JS library