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/prompt_command.rs

@@ -1,7 +1,9 @@
-use super::{SlashCommand, SlashCommandOutput};
 use crate::prompt_library::PromptStore;
 use anyhow::{anyhow, Context, Result};
-use assistant_slash_command::{ArgumentCompletion, SlashCommandOutputSection};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
 use gpui::{Task, WeakView};
 use language::{BufferSnapshot, LspAdapterDelegate};
 use std::sync::{atomic::AtomicBool, Arc};
@@ -61,7 +63,7 @@ impl SlashCommand for PromptSlashCommand {
         _workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         cx: &mut WindowContext,
-    ) -> Task<Result<SlashCommandOutput>> {
+    ) -> Task<SlashCommandResult> {
         let title = arguments.to_owned().join(" ");
         if title.trim().is_empty() {
             return Task::ready(Err(anyhow!("missing prompt name")));