Custom matrix lite support for split keyboards (#14674)

* Custom matrix lite support for split keyboards

* WIP: matrix -> matrix_common refactor

* Move matrix_post_scan() to matrix_common.c

quantum/matrix.h

@@ -73,6 +73,7 @@ void matrix_init_user(void);
 void matrix_scan_user(void);
 
 #ifdef SPLIT_KEYBOARD
+bool matrix_post_scan(void);
 void matrix_slave_scan_kb(void);
 void matrix_slave_scan_user(void);
 #endif