add a `file_all` function

[elong0527]

Should we add a file_all function?

It would be helpful if we need to move all files from a sub-folder. (e.g. inst/ folder)

[nanxstats]

This is what file_auto() is supposed to do.

We just need to implement this https://github.com/Merck/pkglite/issues/18 to guess the type of files with unknown extensions.