New check: Warn when errors.Is() arguments swapped

[mholt]

It's almost never right to write errors.Is(fs.ErrNotExist, err), but I still do it sometimes to much head-scratching 😅 It should be errors.Is(err, fs.ErrNotExist).

Would love it if there's way to warn when the first argument is an exported error value, especially one in another package... catch it much sooner.

Thanks for the consideration!

Also, many thanks for Staticcheck. It is a huge timesaver.