[feature request] Make an `.active()` function to retreive the current open notifications.

[javalsai]

Pretty much what the title says, this would allow to better integrate with currently displaying notifications.

It would be useful for example, in my case I want to keymap .dismiss()ing a notification only if there's one, if not fall back to another function. This allows me to.

Already played with the codebase and this fork should be enough: https://github.com/javalsai/nvim-notify-patch. Idk the lib internals and barely any lua. Didn't get :lenght() to work so that's why I wrote it like that, its likely improvable.