Introduce /search command to assistant (#12372)

This pull request introduces semantic search to the assistant using a
slash command:


https://github.com/zed-industries/zed/assets/482957/62f39eae-d7d5-46bf-a356-dd081ff88312

Moreover, this also adds a status to pending slash commands, so that we
can show when a query is running or whether it failed:

<img width="1588" alt="image"
src="https://github.com/zed-industries/zed/assets/482957/e8d85960-6275-4552-a068-85efb74cfde1">

I think this could be better design-wise, but seems like a pretty good
start.

Release Notes:

- N/A

crates/assistant/src/assistant.rs

@@ -16,12 +16,14 @@ use command_palette_hooks::CommandPaletteFilter;
 pub(crate) use completion_provider::*;
 use gpui::{actions, AppContext, Global, SharedString, UpdateGlobal};
 pub(crate) use saved_conversation::*;
+use semantic_index::{CloudEmbeddingProvider, SemanticIndex};
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore};
 use std::{
     fmt::{self, Display},
     sync::Arc,
 };
+use util::paths::EMBEDDINGS_DIR;
 
 actions!(
     assistant,
@@ -232,6 +234,21 @@ impl Assistant {
 pub fn init(client: Arc<Client>, cx: &mut AppContext) {
     cx.set_global(Assistant::default());
     AssistantSettings::register(cx);
+
+    cx.spawn(|mut cx| {
+        let client = client.clone();
+        async move {
+            let embedding_provider = CloudEmbeddingProvider::new(client.clone());
+            let semantic_index = SemanticIndex::new(
+                EMBEDDINGS_DIR.join("semantic-index-db.0.mdb"),
+                Arc::new(embedding_provider),
+                &mut cx,
+            )
+            .await?;
+            cx.update(|cx| cx.set_global(semantic_index))
+        }
+    })
+    .detach();
     completion_provider::init(client, cx);
     assistant_slash_command::init(cx);
     assistant_panel::init(cx);