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/diagnostics_command.rs

@@ -1,6 +1,8 @@
-use super::{create_label_for_command, SlashCommand, SlashCommandOutput};
 use anyhow::{anyhow, Result};
-use assistant_slash_command::{ArgumentCompletion, SlashCommandOutputSection};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+    SlashCommandResult,
+};
 use fuzzy::{PathMatch, StringMatchCandidate};
 use gpui::{AppContext, Model, Task, View, WeakView};
 use language::{
@@ -19,6 +21,8 @@ use util::paths::PathMatcher;
 use util::ResultExt;
 use workspace::Workspace;
 
+use crate::slash_command::create_label_for_command;
+
 pub(crate) struct DiagnosticsSlashCommand;
 
 impl DiagnosticsSlashCommand {
@@ -167,7 +171,7 @@ impl SlashCommand for DiagnosticsSlashCommand {
         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!("workspace was dropped")));
         };