feat: use vim.notify for user notifications

[emmanueltouzery]

fixes #473
vim.notify allows the user to customize the display of messages

I'm not sure about the schedule parameter, whether it's still needed. I left it in just in case.

The testing I performed is that I triggered the:

"The file is open with unsaved changes! Aborting file restoration."

message, by modifying a file, saving, modifying again, then asking to revert the changes in diffview, that triggered that message. I also tried modifying the location where the message is triggered to ask to trigger it with the schedule parameter being "true" and that worked too.

[sindrets]

Just remove that one early return, and this looks good!

And, yes, let's keep the schedule parameter. It's useful for two reasons:

1. When these are called from an async context. Without scheduling the notifications are likely to fail when they try to access the nvim API.
2. In performance critical code it's useful for deferring the handling of the notification until later.

[emmanueltouzery]

I force pushed the removal the nil-check now. I had it in because the previous code also had it (well, it had if not msg[1]). Thank you for the explanation for schedule!

[emmanueltouzery]

my bad about the type signature 😬
force pushed the fix now.

[sindrets]

LGTM, thank you!