Refactor/main menu (#416)

* Add basic common_ui background gfx functions

* Move game main menu to separate file

* clang format

* tmp commit basic docs

* clang format

* scope vars and cleanup

* Quick consistency update. Finalize.

This branch is based on: https://github.com/GBALATRO/balatro-gba/pull/363
Co-authored-by: emiyl <me@emiyl.com>

* Simplify global struct

* clang-format

* Add clang-format to subdirs

* Move timer definitions into their own file

* clang-format

* Fix action yml

* docs

* Update for PR

* Move game start to a state

* Remove lag switch

* Remove GameVariables pointer for callbacks

* Make game_vars a global

* move declaration of game vars in header

* Fix multiple game_vars

* fix for pr

* remove change_background in update again

* clang format

* Make bgCallbacks table static const

---------

Co-authored-by: emiyl <me@emiyl.com>

include/game.h

@@ -6,6 +6,8 @@
 
 #include <tonc.h>
 
+extern GameVariables g_game_vars;
+
 #define MAX_HAND_SIZE        16
 #define MAX_DECK_SIZE        52
 #define MAX_JOKERS_HELD_SIZE 5 // This doesn't account for negatives right now.
@@ -172,6 +174,8 @@ void set_game_speed(int new_game_speed);
 bool is_shortcut_joker_active(void);
 int get_straight_and_flush_size(void);
 
+void game_start(void);
+
 // Temporary change for Refactor. Currently this compatibility binder is to allow
 // simultaneous integration of the new system in `common_ui` with the the existing
 // old system incrementally and without losing functionality.