feat(--headless): add on_print callback to stdioopen

This commit adds an on_print callback to stdioopen's dictionary
argument which lets the caller specify a function called each time
neovim will try to output something to stdout (e.g. on "echo" or
"echoerr" in --headless mode).

src/nvim/eval/typval.h

@@ -81,7 +81,8 @@ typedef struct {
   } data;
   CallbackType type;
 } Callback;
-#define CALLBACK_NONE ((Callback){ .type = kCallbackNone })
+#define CALLBACK_INIT { .type = kCallbackNone }
+#define CALLBACK_NONE ((Callback)CALLBACK_INIT)
 
 /// Structure holding dictionary watcher
 typedef struct dict_watcher {