Add a SlashCommandResult type alias (#19633)

This PR adds a new `SlashCommandResult` type alias.

We're going to be changing what slash commands can return in order to
support streaming, so having this type alias in place will make that
switch a bit more neat.

Release Notes:

- N/A

crates/extension/src/extension_slash_command.rs

@@ -3,6 +3,7 @@ use std::sync::{atomic::AtomicBool, Arc};
 use anyhow::{anyhow, Result};
 use assistant_slash_command::{
     ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
 };
 use futures::FutureExt;
 use gpui::{Task, WeakView, WindowContext};
@@ -87,7 +88,7 @@ impl SlashCommand for ExtensionSlashCommand {
         _workspace: WeakView<Workspace>,
         delegate: Option<Arc<dyn LspAdapterDelegate>>,
         cx: &mut WindowContext,
-    ) -> Task<Result<SlashCommandOutput>> {
+    ) -> Task<SlashCommandResult> {
         let arguments = arguments.to_owned();
         let output = cx.background_executor().spawn(async move {
             self.extension