assistant: Add diagnostics slash command (#12998)

This adds a `diagnostics` command to the assistant which allows to
inject compile errors/warnings into the context.

Release Notes:

- N/A

crates/assistant/src/slash_command.rs

@@ -18,6 +18,7 @@ use workspace::Workspace;
 
 pub mod active_command;
 pub mod default_command;
+pub mod diagnostics_command;
 pub mod fetch_command;
 pub mod file_command;
 pub mod now_command;