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/tab_command.rs

@@ -1,6 +1,8 @@
-use super::{file_command::append_buffer_to_output, SlashCommand, SlashCommandOutput};
 use anyhow::{Context, Result};
-use assistant_slash_command::{ArgumentCompletion, SlashCommandOutputSection};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
 use collections::{HashMap, HashSet};
 use editor::Editor;
 use futures::future::join_all;
@@ -14,6 +16,8 @@ use ui::{ActiveTheme, WindowContext};
 use util::ResultExt;
 use workspace::Workspace;
 
+use crate::slash_command::file_command::append_buffer_to_output;
+
 pub(crate) struct TabSlashCommand;
 
 const ALL_TABS_COMPLETION_ITEM: &str = "all";
@@ -132,7 +136,7 @@ impl SlashCommand for TabSlashCommand {
         workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         cx: &mut WindowContext,
-    ) -> Task<Result<SlashCommandOutput>> {
+    ) -> Task<SlashCommandResult> {
         let tab_items_search = tab_items_for_queries(
             Some(workspace),
             arguments,