Export pkg_file_lua and promote common organisation of lua filters in packages

[cderv]

Following #1899 (comment), 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.

[github-actions]

This old thread has been automatically locked. If you think you have found something related to this, please open a new issue by following the issue guide (https://yihui.org/issue/), and link to this old issue if necessary.