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/language/src/language.rs

@@ -72,7 +72,7 @@ pub use language_registry::{
 pub use lsp::LanguageServerId;
 pub use outline::{Outline, OutlineItem};
 pub use syntax_map::{OwnedSyntaxLayer, SyntaxLayer};
-pub use text::LineEnding;
+pub use text::{AnchorRangeExt, LineEnding};
 pub use tree_sitter::{Node, Parser, Tree, TreeCursor};
 
 /// Initializes the `language` crate.