fix(messages)!: vim.ui_attach message callbacks are unsafe

Problem:  Lua callbacks for "msg_show" events with vim.ui_attach() are
          executed when it is not safe.
Solution: Disallow non-fast API calls for "msg_show" event callbacks.
          Automatically detach callback after excessive errors.
          Make sure fast APIs do not modify Nvim state.

src/nvim/eval.c

@@ -8616,7 +8616,7 @@ bool eval_has_provider(const char *feat, bool throw_if_fast)
   }
 
   if (throw_if_fast && !nlua_is_deferred_safe()) {
-    semsg(e_luv_api_disabled, "Vimscript function");
+    semsg(e_fast_api_disabled, "Vimscript function");
     return false;
   }