add a notification, when a new version is available, either at startup, or in chat when joining a world

For Forge, it may be worth adding a updateJSONURL in mods.toml to add a notification to the "Mods" button alongside any other notification options.

Since Vivecraft is already up on Modrinth, they provide a service for this so you can just add the URL to the mods.toml and let them do the work of hosting and generating the JSON file.