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/search_command.rs

@@ -1,10 +1,8 @@
-use super::{
-    create_label_for_command,
-    file_command::{build_entry_output_section, codeblock_fence_for_path},
-    SlashCommand, SlashCommandOutput,
-};
 use anyhow::Result;
-use assistant_slash_command::{ArgumentCompletion, SlashCommandOutputSection};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
 use feature_flags::FeatureFlag;
 use gpui::{AppContext, Task, WeakView};
 use language::{CodeLabel, LspAdapterDelegate};
@@ -16,6 +14,9 @@ use std::{
 use ui::{prelude::*, IconName};
 use workspace::Workspace;
 
+use crate::slash_command::create_label_for_command;
+use crate::slash_command::file_command::{build_entry_output_section, codeblock_fence_for_path};
+
 pub(crate) struct SearchSlashCommandFeatureFlag;
 
 impl FeatureFlag for SearchSlashCommandFeatureFlag {
@@ -63,7 +64,7 @@ impl SlashCommand for SearchSlashCommand {
         workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         cx: &mut WindowContext,
-    ) -> Task<Result<SlashCommandOutput>> {
+    ) -> Task<SlashCommandResult> {
         let Some(workspace) = workspace.upgrade() else {
             return Task::ready(Err(anyhow::anyhow!("workspace was dropped")));
         };