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/main.c

@@ -10,7 +10,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <time.h>
 
 #include "auto/config.h"
 #include "nvim/arglist.h"
@@ -62,7 +61,6 @@
 #include "nvim/os/fileio.h"
 #include "nvim/os/input.h"
 #include "nvim/os/os.h"
-#include "nvim/os/pty_process.h"
 #include "nvim/os/stdpaths_defs.h"
 #include "nvim/os/time.h"
 #include "nvim/path.h"
@@ -97,7 +95,6 @@
 #include "nvim/msgpack_rpc/helpers.h"
 #include "nvim/msgpack_rpc/server.h"
 #include "nvim/os/signal.h"
-#include "nvim/tui/tui.h"
 
 // values for "window_layout"
 enum {