assistant: Add /fetch slash command (#12645)

This PR adds a new `/fetch` slash command to the Assistant for fetching
the content of an arbitrary URL as Markdown.

Currently it's just using the same HTML to Markdown conversion that
`/rustdoc` uses, but I'll be working to refine the output to be more
widely useful.

Release Notes:

- N/A

crates/assistant/src/slash_command.rs

@@ -17,6 +17,7 @@ use std::{
 use workspace::Workspace;
 
 pub mod active_command;
+pub mod fetch_command;
 pub mod file_command;
 pub mod project_command;
 pub mod prompt_command;