Refactor task handling to use VecDeque references

- Change `get_tasks` to return `&VecDeque<Tasks>` instead of a formatted
  `String`.
- Introduce `get_tasks_formatted` for display purposes.
- Update `MinerUnit`, `Unit` trait, `Units` enum, and its `Option`
  implementation accordingly.
- Add `EMPTY_TASKS` lazy static for handling empty task lists.
- Adjust `BoardState` to use the new task API (commented out
  conditional).

src/app/states/skirmish_states/board.rs

@@ -219,11 +219,14 @@ impl BoardState {
         }
 
         let (row, col) = self.marked_cells.selected_unit.get_coords();
+
+        // if !self.marked_cells.marked_cells.len() == 1 {
         let task: Tasks = Tasks::Digging(DiggingTask::new(self.marked_cells.marked_cells.clone()));
 
         self.get_mut_cell(row, col)
             .get_mut_option_unit()
             .set_task(task);
+        // }
 
         self.marked_cells.selected_unit = None;
         self.marked_cells.marked_cells.clear();