Export pkg_file_lua and promote common organisation of lua filters in packages

Following https://github.com/rstudio/rmarkdown/pull/1899#discussion_r492834759, we could export `pkg_file_lua` so that there is a helper that goes with a common location of lua filters inside packages. 

`inst/rmarkdow/lua` would be a good place.
