Add support for NotifyBackground

[matadaniel]

Thank you for the effort you've put into this awesome color scheme!

I'm using the following options and get the following warning.

sonokai_style = "maia"
sonokai_enable_italic = 1
sonokai_transparent_background = 2
sonokai_current_word = "grey background"

[antoineco]

Good catch 👍

I haven't been using nvim-notify but it seems like we need to set a few more groups:

https://github.com/rcarriga/nvim-notify/blob/7138c86b28de61b6866c8cae60289136f0d539fa/lua/notify/config/highlights.lua#L5-L28