Warning "Directory already exists" when knitting Rmds after the first time

This line unconditionally tries to create the temporary pandoc library directory, which persists after knitting one Rmd from within PyCharm.
This results in a warning being displayed on every knit operation.
Guarding this clause with `if (!dir.exists(library_path))` will prevent the warning, leading to cleaner console output.

https://github.com/JetBrains/Rkernel/blob/b270cc0dc8261ebfa35a63d7c02a68ff6fc40c54/R/render_markdown.R#L110