make FS predicates compatible with Scryer library(files)

[guregu]

See: 6a491d2#r69581698

Scryer's library(files) uses strings instead of atoms, let's try to match compatibility as close as possible.

I'm not sure if I should change how list handling in consult/1 works 🤔

It might be handy to have consult/1 handle lists as strings, then you could write something like directory_files(some_dir, Files), maplist(consult, Files). However, taking a peek at Scryer it seems to not accept lists at all. I would imagine that's the standard. ichiban/prolog accepts lists like SWI, so it could be confusing to provide a replacement that treats lists differently. Hmm..

[guregu]

This change just landed in v0.0.5. More filesystem predicates coming soon!

I decided to keep consult/1 as-is for now, but opinions are welcome.