Supports `<local path>/.config/helix/...` in addition to `<local path>/.helix/...`

[poliorcetics]

I initially mentionned it here #1249 (comment), but I did not argue a lot for it at the time because I wanted the initial functionnality more.

In retrospect, that was probably a mistake, because now helix is another bad citizen when it could have been a nice one from the start.

Prior art is cargo-nextest (and the author's recommandations), as well the XDG_CONFIG_HOME env var and the XDG spec in general, see the Command Line Interface Guidelines