Add icon for AC mode

[fvincenzo]

If tmux theme is run on a desktop the battery space is empty.
Restore the previous behavior displaying AC and an icon.

[Theoreticallyhugo]

heya mate,
that sounds like a good idea.
however, i'd love to see some configurability allowing users to use dracula without nerd-fonts too.
while i prefer nerdfonts, i think being able to use it without them is important.
(if youre not using a nerd-font symbol and its just github being unable to display it, i apologise and approve your changes)