Add a slash to directories when listing

Having a slash after a directory name is yelling *DIRECTORY* for decades.

That's why I really like the -F flag.

It has other useful features as well.

From the `ls` manual:

> Display a slash (`/') immediately after each pathname that is a directory, an asterisk (`*') after each that is executable, an at sign (`@')
> after each symbolic link, an equals sign (`=') after each socket, a percent sign (`%') after each whiteout, and a vertical bar (`|') after each that is a
> FIFO.