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/assistant.rs

@@ -25,8 +25,8 @@ use semantic_index::{CloudEmbeddingProvider, SemanticIndex};
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};
 use slash_command::{
-    active_command, file_command, project_command, prompt_command, rustdoc_command, search_command,
-    tabs_command,
+    active_command, fetch_command, file_command, project_command, prompt_command, rustdoc_command,
+    search_command, tabs_command,
 };
 use std::{
     fmt::{self, Display},
@@ -304,6 +304,7 @@ fn register_slash_commands(cx: &mut AppContext) {
     slash_command_registry.register_command(project_command::ProjectSlashCommand, true);
     slash_command_registry.register_command(search_command::SearchSlashCommand, true);
     slash_command_registry.register_command(rustdoc_command::RustdocSlashCommand, false);
+    slash_command_registry.register_command(fetch_command::FetchSlashCommand, false);
 
     let store = PromptStore::global(cx);
     cx.background_executor()