Add --error-format flag to serialize err diagnostics

[yannham]

Closes #1738.

This PR adds an --error-format global flag (copied over from Rust, after benchmarking what a few other compilers are donig, this seemed to be the better tradeoff between being intuitive and not conflicting with other meaning of "format as JSON", which we have a lot in Nickel - export format, input format, etc.).

The user can choose between JSON, TOML or YAML - I believe JSON is the most common one, but it's virtually free to support more formats with serde (at the least the format that we export Nickel configurations to), hence I don't see an obvious reason to not support them.

Incidentally, this PR formats more CLI errors using codespan diagnostics (they were just printed as text before), so that the flag really affects all error messages. As a bonus, those error messages are now nicely colored like the others.

[jneem]

Does this imply any stability guarantees for the error messages? I'd imagine that if people are consuming them programmatically then they're expecting at least part of the format to be stable...

[yannham]

Does this imply any stability guarantees for the error messages? I'd imagine that if people are consuming them programmatically then they're expecting at least part of the format to be stable...

Good question. I would be tempted to say "no", because it makes our life simpler 😛 and also because currently it's just serializing the library datatype that we happen to use (codespan_reporting). It's probably not too hard to convert it if we ever change the error reporting infrastructure, but still. In the meantime, I should probably add a mention in the documentation of the CLI argument that the format isn't stable.