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/context/context_tests.rs

@@ -6,7 +6,7 @@ use crate::{
 use anyhow::Result;
 use assistant_slash_command::{
     ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
-    SlashCommandRegistry,
+    SlashCommandRegistry, SlashCommandResult,
 };
 use collections::HashSet;
 use fs::FakeFs;
@@ -1416,7 +1416,7 @@ impl SlashCommand for FakeSlashCommand {
         _workspace: WeakView<Workspace>,
         _delegate: Option<Arc<dyn LspAdapterDelegate>>,
         _cx: &mut WindowContext,
-    ) -> Task<Result<SlashCommandOutput>> {
+    ) -> Task<SlashCommandResult> {
         Task::ready(Ok(SlashCommandOutput {
             text: format!("Executed fake command: {}", self.0),
             sections: vec![],