Use nvim-notify instead of default toasts

[alexmozaidze]

There are a lot of plugins utilizing the power of configurable and fully controllable notifications with the help of nvim-notify, and I think it wouldn't be unwise to use it instead of the default toast system.

The dependency would be optional, in that Neogit will detect if nvim-notify exists, and if not, the toast system will be used as a fallback.

NOTE: Keep in mind that I am no developer, it's just to see what majority prefers.

[gegoune]

Set disable_builtin_notifications to true and just use vim.notify as you like.