* Remove `mlua` from the `dev-dependencies`
* Test `api::notify()`
* Fix `api::notify()`'s API
* Test `api::notify()` w/ custom provider
* Add an `Arena` argument to `nvim_notify` on 0.10 and Nightly
* Directly implement `Error` for `types::Error`
* Test that errors returned by `vim.notify` are propagated
* Fix using `mlua` in CI
* Don't test `notify_custom{_err}` on `v0.9.5` on macOS and Windows