Custom Toast Notifications

A command like /toast would be great. Right now, we can make custom advancements to show toast notifications, but that's complicated and annoying. An advancement can only have one name so you can't use it for customizable messages. With JSON formatting like /tellraw, you can put in player names based on scoreboards which would be really useful for datapacks.

I think the toast notifications used for advancements and recipes have potential to be really useful. Sometimes, you want to notify players about something but it isn't something you want to stick around in their chat. You just want to tell them maybe something like "[player] is sleeping" or something. A message like that shouldn't have to stay in chat. You should be able to send non-disruptive messages. The action bar is great for this but sometimes you're using the action bar for other things and the action bar often isn't very readable.