Move to using annotate-snippets-rs for producing snippets

[Manishearth]

@zbraniecki created https://github.com/zbraniecki/annotate-snippets-rs which basically does Rust-style formatting of code snippets for errors, to be used within Fluent.

I think we should try and move our snippet code out of tree, preferably merging it with annotate-snippets-rs (which is already close to what rustc does), so that it's reusable.

@zbraniecki has agreed to make whatever changes necessary to the crate to make it usable within rustc.

cc @estebank

[oli-obk]

cc @rust-lang/wg-diagnostics