refactor: fix IWYU mapping file and use IWYU (#21802)

Also add the EXITFREE definition to main_lib rather than the nvim target, as the header generation needs the EXITFREE flag to work properly.

src/nvim/indent_c.c

@@ -23,7 +23,6 @@
 #include "nvim/pos.h"
 #include "nvim/search.h"
 #include "nvim/strings.h"
-#include "nvim/types.h"
 #include "nvim/vim.h"
 
 // Find result cache for cpp_baseclass