Add mission briefing menu in pause menu

[cxong]

No description provided.

[cxong]

The pause menu is created in pause_menu.c. This task will involve adding a new submenu, named "Briefing", which will display the mission description, and go back to the previous screen on inputting button1 or button2. Disable the menu if the mission description is null or empty. Use the MENU_TYPE_BASIC menu type, followed by custom draw and update callbacks to draw the briefing text and go back to the previous menu respectively.