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/extension/src/extension_slash_command.rs

@@ -1,6 +1,6 @@
 use crate::wasm_host::{WasmExtension, WasmHost};
 use anyhow::{anyhow, Result};
-use assistant_slash_command::{SlashCommand, SlashCommandOutput};
+use assistant_slash_command::{SlashCommand, SlashCommandOutput, SlashCommandOutputSection};
 use futures::FutureExt;
 use gpui::{AppContext, IntoElement, Task, WeakView, WindowContext};
 use language::LspAdapterDelegate;
@@ -49,7 +49,7 @@ impl SlashCommand for ExtensionSlashCommand {
         cx: &mut WindowContext,
     ) -> Task<Result<SlashCommandOutput>> {
         let argument = argument.map(|arg| arg.to_string());
-        let output = cx.background_executor().spawn(async move {
+        let text = cx.background_executor().spawn(async move {
             let output = self
                 .extension
                 .call({
@@ -76,12 +76,16 @@ impl SlashCommand for ExtensionSlashCommand {
             output.ok_or_else(|| anyhow!("no output from command: {}", self.command.name))
         });
         cx.foreground_executor().spawn(async move {
-            let output = output.await?;
+            let text = text.await?;
+            let range = 0..text.len();
             Ok(SlashCommandOutput {
-                text: output,
-                render_placeholder: Arc::new(|_, _, _| {
-                    "TODO: Extension command output".into_any_element()
-                }),
+                text,
+                sections: vec![SlashCommandOutputSection {
+                    range,
+                    render_placeholder: Arc::new(|_, _, _| {
+                        "TODO: Extension command output".into_any_element()
+                    }),
+                }],
             })
         })
     }