lua: add vim.register_keystroke_callback (#12536)

* feat: Add vim.register_keystroke_callback

* fixup: Forgot to remove mention of old option

* fixup: Answer jamessan comments

* fixup: Answer norcalli comments

* fixup: portability

* Update runtime/doc/lua.txt

Co-authored-by: Ashkan Kiani <ashkan.k.kiani@gmail.com>

src/nvim/getchar.c

@@ -27,6 +27,7 @@
 #include "nvim/ex_docmd.h"
 #include "nvim/ex_getln.h"
 #include "nvim/func_attr.h"
+#include "nvim/lua/executor.h"
 #include "nvim/main.h"
 #include "nvim/mbyte.h"
 #include "nvim/memline.h"
@@ -1535,6 +1536,9 @@ int vgetc(void)
    */
   may_garbage_collect = false;
 
+  // Exec lua callbacks for on_keystroke
+  nlua_execute_log_keystroke(c);
+
   return c;
 }