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/mapping.c

@@ -5,6 +5,7 @@
 
 #include <assert.h>
 #include <inttypes.h>
+#include <lauxlib.h>
 #include <limits.h>
 #include <stdbool.h>
 #include <stdio.h>