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_slash_command/src/assistant_slash_command.rs

@@ -4,7 +4,10 @@ use anyhow::Result;
 use gpui::{AnyElement, AppContext, ElementId, Task, WeakView, WindowContext};
 use language::LspAdapterDelegate;
 pub use slash_command_registry::*;
-use std::sync::{atomic::AtomicBool, Arc};
+use std::{
+    ops::Range,
+    sync::{atomic::AtomicBool, Arc},
+};
 use workspace::Workspace;
 
 pub fn init(cx: &mut AppContext) {
@@ -44,5 +47,11 @@ pub type RenderFoldPlaceholder = Arc<
 
 pub struct SlashCommandOutput {
     pub text: String,
+    pub sections: Vec<SlashCommandOutputSection<usize>>,
+}
+
+#[derive(Clone)]
+pub struct SlashCommandOutputSection<T> {
+    pub range: Range<T>,
     pub render_placeholder: RenderFoldPlaceholder,
 }