Make fetch slash command visible in the command selector (#21302)

The `/fetch` command is naturally already accessible via the completion
menu when you type / in the assistant panel, but it wasn't on the "Add
Context" command selector. I think it should! It's a super nice/powerful
one, and I've seen folks not knowing it existed. Side-note: maybe, in
the near future, it'd be best to rename it to "`/web`, as that's an
easier name to parse and assume what it does.

<img width="700" alt="Screenshot 2024-11-28 at 16 52 07"
src="https://github.com/user-attachments/assets/37134e1c-c788-48ca-81ae-d7416e8c8706">

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -342,8 +342,7 @@ fn register_slash_commands(prompt_builder: Option<Arc<PromptBuilder>>, cx: &mut
     slash_command_registry.register_command(terminal_command::TerminalSlashCommand, true);
     slash_command_registry.register_command(now_command::NowSlashCommand, false);
     slash_command_registry.register_command(diagnostics_command::DiagnosticsSlashCommand, true);
-    slash_command_registry.register_command(fetch_command::FetchSlashCommand, false);
-    slash_command_registry.register_command(fetch_command::FetchSlashCommand, false);
+    slash_command_registry.register_command(fetch_command::FetchSlashCommand, true);
 
     if let Some(prompt_builder) = prompt_builder {
         cx.observe_flag::<project_command::ProjectSlashCommandFeatureFlag, _>({

crates/assistant/src/slash_command/fetch_command.rs

@@ -108,6 +108,10 @@ impl SlashCommand for FetchSlashCommand {
         "Insert fetched URL contents".into()
     }
 
+    fn icon(&self) -> IconName {
+        IconName::Globe
+    }
+
     fn menu_text(&self) -> String {
         self.description()
     }
@@ -162,7 +166,7 @@ impl SlashCommand for FetchSlashCommand {
                 text,
                 sections: vec![SlashCommandOutputSection {
                     range,
-                    icon: IconName::AtSign,
+                    icon: IconName::Globe,
                     label: format!("fetch {}", url).into(),
                     metadata: None,
                 }],