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/context_server_command.rs

@@ -1,8 +1,7 @@
-use super::create_label_for_command;
 use anyhow::{anyhow, Result};
 use assistant_slash_command::{
     AfterCompletion, ArgumentCompletion, SlashCommand, SlashCommandOutput,
-    SlashCommandOutputSection,
+    SlashCommandOutputSection, SlashCommandResult,
 };
 use collections::HashMap;
 use context_servers::{
@@ -17,6 +16,8 @@ use text::LineEnding;
 use ui::{IconName, SharedString};
 use workspace::Workspace;
 
+use crate::slash_command::create_label_for_command;
+
 pub struct ContextServerSlashCommand {
     server_id: String,
     prompt: Prompt,
@@ -128,7 +129,7 @@ impl SlashCommand for ContextServerSlashCommand {
         _workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         cx: &mut WindowContext,
-    ) -> Task<Result<SlashCommandOutput>> {
+    ) -> Task<SlashCommandResult> {
         let server_id = self.server_id.clone();
         let prompt_name = self.prompt.name.clone();