Prepare release candidates

We'll use "-rcX" instead of "-pre" to allow multiple ones, jic
Additionally, they will be able to be detected using __RGBDS_RC__
Finally, adapt everything version-related to this new system

include/version.h

@@ -12,6 +12,7 @@
 #define PACKAGE_VERSION_MAJOR 0
 #define PACKAGE_VERSION_MINOR 4
 #define PACKAGE_VERSION_PATCH 2
+// #define PACKAGE_VERSION_RC 1
 
 const char *get_package_version_string(void);