Decide on convention for generated code

[lpil]

Codegen is often useful! We don't currently have any recommendations for how it should be done, but we could start with a convention where generated code should be written to and how to make it clear that it is generated.

To avoid accidental editing we could copy rust-analyzer's handling of cfg'd out code and use the unnecessary tag to fade out generated code.

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#diagnosticTag