rustdoc: Index crates progressively (#13011)

This PR updates the rustdoc indexing to be more progressive.

Rather than waiting until we've crawled the entire crate to begin
writing to the database, we instead start writing the docs as we go.

This makes it so you can start getting completions while the indexing is
still running.

Release Notes:

- N/A

crates/assistant/src/slash_command/rustdoc_command.rs

@@ -10,7 +10,7 @@ use gpui::{AppContext, Model, Task, WeakView};
 use http::{AsyncBody, HttpClient, HttpClientWithUrl};
 use language::LspAdapterDelegate;
 use project::{Project, ProjectPath};
-use rustdoc::crawler::LocalProvider;
+use rustdoc::LocalProvider;
 use rustdoc::{convert_rustdoc_to_markdown, RustdocStore};
 use ui::{prelude::*, ButtonLike, ElevationIndex};
 use workspace::Workspace;