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/cargo_workspace_command.rs

@@ -1,6 +1,8 @@
-use super::{SlashCommand, SlashCommandOutput};
 use anyhow::{anyhow, Context, Result};
-use assistant_slash_command::{ArgumentCompletion, SlashCommandOutputSection};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
 use fs::Fs;
 use gpui::{AppContext, Model, Task, WeakView};
 use language::{BufferSnapshot, LspAdapterDelegate};
@@ -123,7 +125,7 @@ impl SlashCommand for CargoWorkspaceSlashCommand {
         workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         cx: &mut WindowContext,
-    ) -> Task<Result<SlashCommandOutput>> {
+    ) -> Task<SlashCommandResult> {
         let output = workspace.update(cx, |workspace, cx| {
             let project = workspace.project().clone();
             let fs = workspace.project().read(cx).fs().clone();