Support translating the compiler diagnostics

[Manishearth]

This is mostly a tracking issue, the actual implementation of this should be done working with the compiler and release team. But feel free to discuss the overall plan here.

Previous discussion: https://internals.rust-lang.org/t/translating-the-compiler/10376

Blocked on rust-lang/rust#61132

Once we have that implemented as a custom derive , we need to:

• Provide a way to produce separate Fluent (or other format) files based on these diagnostics. This can either be done by making the compiler build produce auxiliary files (by having the proc macro be impure), or by writing a separate tool that finds and parses these
• Sync these to a separate repo, hooked up to Pontoon, where they can be translated
• Allow the compiler to use the Fluent strings picked up from the sysroot instead
• As a part of the release process of Rust, allow language packs containing these strings to be installed via rustup
• Allow the compiler to pick up the "default" language from a rustup setting

[Manishearth]

cc @estebank

[davidtwco]

Just learnt about this issue: work has started on this, there's a document summarizing the effort and a document with a list of all pull requests contributing to the effort.