assistant: Show an indicator when a crate is being indexed (#13174)

This PR adds an indicator when a crate is being indexed as part of the
`/rustdoc` command invocation.


https://github.com/zed-industries/zed/assets/1486634/0dd4b663-658c-4be5-a342-cfbd7a938fca

Release Notes:

- N/A

crates/rustdoc/src/store.rs

@@ -71,6 +71,11 @@ impl RustdocStore {
         }
     }
 
+    /// Returns whether the crate with the given name is currently being indexed.
+    pub fn is_indexing(&self, crate_name: &CrateName) -> bool {
+        self.indexing_tasks_by_crate.read().contains_key(crate_name)
+    }
+
     pub async fn load(
         &self,
         crate_name: CrateName,