Use `vim.notify` as default console log

[lettertwo]

vim.notify is arguably a more ergonomic way of showing level-filtered messages to the user, and it also allows for custom providers and handling, like nvim-notify or noice , to integrate messages from plenary.log.

I'm not sure if it's a bad idea to make this the new default; it could certainly be a breaking change, but i think that the intention of an author who has not overridden the default config probably aligns with the behavior of vim.notify.

Fixes #540