feat(float): implement footer

Problem: Now way to show text at the bottom part of floating window
  border (a.k.a. "footer").

Solution: Allows `footer` and `footer_pos` config fields similar to
  `title` and `title_pos`.

src/nvim/window.c

@@ -5239,8 +5239,9 @@ static void win_free(win_T *wp, tabpage_T *tp)
     }
   }
 
-  // free the border title text
+  // free the border text
   clear_virttext(&wp->w_float_config.title_chunks);
+  clear_virttext(&wp->w_float_config.footer_chunks);
 
   clear_matches(wp);