There are 3 repositories under rstudio-addins topic.
RStudio Addins and Shiny Modules for Medical Research
RStudio addins to prettify JavaScript/HTML/CSS/Markdown/C++ and more.
RStudio addin for formatting R source code with formatR
Lecture about R package
RStudio add-ins for manipulating strings
Wilson's Useful and Awesome RStudio ADDins