added mac keybinding for backward-delete-char

lib/key-bindings.zsh

@@ -21,6 +21,7 @@ bindkey ' ' magic-space    # also do history expansion on space
 bindkey '^[[Z' reverse-menu-complete
 
 # Make the delete key (or Fn + Delete on the Mac) work instead of outputting a ~
+bindkey '^?' backward-delete-char
 bindkey "^[[3~" delete-char
 bindkey "^[3;5~" delete-char
 bindkey "\e[3~" delete-char