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>

.github/workflows/clang_format_ci_workflow.yml

@@ -17,4 +17,4 @@ jobs:
         uses: actions/checkout@v6
 
       - name: Run clang-format
-        run: clang-format --dry-run -Werror include/*.h source/*.c
+        run: clang-format --dry-run -Werror include/*.h source/*.c include/game/*.h source/game/*.c