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/assistant/src/slash_command/now_command.rs

@@ -4,6 +4,7 @@ use std::sync::Arc;
 use anyhow::Result;
 use assistant_slash_command::{
     ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
 };
 use chrono::Local;
 use gpui::{Task, WeakView};
@@ -48,7 +49,7 @@ impl SlashCommand for NowSlashCommand {
         _workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         _cx: &mut WindowContext,
-    ) -> Task<Result<SlashCommandOutput>> {
+    ) -> Task<SlashCommandResult> {
         let now = Local::now();
         let text = format!("Today is {now}.", now = now.to_rfc2822());
         let range = 0..text.len();