Experimenting with more structured ways to handle command-line input/output in Rust

Emit compilation errors from Fluent source code

Unfortunately this doesn't seem like it can be integrated into rustc's diagnostic system, so has to use miette errors instead. Only the "invalid reference" case is handled for now, and more will need to be added later. There's also a clear need for refactoring in fluent_embed to make everything a bit cleaner.

Created by  finchie  on July 8, 2024
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