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/hashtab.c

@@ -30,6 +30,7 @@
 #include "nvim/hashtab.h"
 #include "nvim/memory.h"
 #include "nvim/message.h"
+#include "nvim/types.h"
 #include "nvim/vim.h"
 
 // Magic value for algorithm that walks through the array.