Deprecated show_file "file" argument in favour of "path"

[radarhere]

First pointed out in #3450 (comment),

Pillow/src/PIL/ImageShow.py

Line 108 in fd2b07c

def show_file(self, file, **options):

the file argument mirrors the builtin file.

This PR deprecates it in favour of path.

[hugovk]

Let's also add it to https://pillow.readthedocs.io/en/stable/deprecations.html#deprecated-features

And can we add a note to https://pillow.readthedocs.io/en/stable/reference/ImageShow.html#PIL.ImageShow.Viewer.show_file as well?

[radarhere]

Ok, done.

[hugovk]

Thanks!