Json output evolution

[epage]

We need to at least have a plan for how to evolve the format

[epage]

I believe it was @Veykril that I talked to at RustNL about how LSP handles this

• cargo test could pass a CLI flag of features requested
• the program can send back the subset that is supported
• instead of namespacing, experimental features put their features (see above) and any custom data into an untyped value under a key named for the feature
  • For features, this allows sub-features