feat: Add callback function to notifications

[tris203]

This commit introduces a new callback function to the notification system. This function can be used to trigger custom events on a notification. It receives a config object and the notification text as parameters. The callback function is optional and is added to the config object in config.lua. The notify function in util.lua has been updated to call the callback function if it is provided.

This would be one way to handle a request we had in https://github.com/tris203/precognition.nvim/issues/62

But open to your ideas about implementation.

Draft as this is just a talking point for now