select.lua: print selection circles before playlist entries

Requested in
mpv-player#14087 (comment) and
mpv-player#15031 (comment)

This is mainly useful to keep highlighting the current playlist entry
after moving the mouse.

player/lua/select.lua

@@ -48,6 +48,8 @@ mp.add_key_binding(nil, "select-playlist", function ()
         if entry.playing then
             default_item = i
         end
+
+        playlist[i] = (entry.playing and "●" or "○") .. " " .. playlist[i]
     end
 
     if #playlist == 0 then