The contents of `variables.h` are moved to `config_variables.h` instead of being included with the preprocessor. The file `variables.h` is removed, so all config variables can be found in a single file instead of being spread over two files without any clear structure. The original declaration order of config variables is preserved. The unnecessary header guard `GAME_VARIABLES_H` from `variables.h` is removed, as the comment `// This file can be included several times.` already serves the same purpose of silencing the header guard check.
A comment is added at the end of `config_variables.h` to mark the location where modders should ideally declare all of their own config variables to avoid merge conflicts with us in the future.
Closes#7472.
Add `scripts/check_config_variables.py` which checks if config variables defined in `src/game/variables.h` and `src/engine/shared/config_variables.h` are unused.
