Allow extensions to suggest packages for /docs completions (#16185)

This PR gives extensions the ability to suggest packages to show up in
`/docs` completions by default.

Release Notes:

- N/A

crates/extension/src/extension_indexed_docs_provider.rs

@@ -31,7 +31,25 @@ impl IndexedDocsProvider for ExtensionIndexedDocsProvider {
     }
 
     async fn suggest_packages(&self) -> Result<Vec<PackageName>> {
-        Ok(Vec::new())
+        self.extension
+            .call({
+                let id = self.id.clone();
+                |extension, store| {
+                    async move {
+                        let packages = extension
+                            .call_suggest_docs_packages(store, id.as_ref())
+                            .await?
+                            .map_err(|err| anyhow!("{err:?}"))?;
+
+                        Ok(packages
+                            .into_iter()
+                            .map(|package| PackageName::from(package.as_str()))
+                            .collect())
+                    }
+                    .boxed()
+                }
+            })
+            .await
     }
 
     async fn index(&self, package: PackageName, database: Arc<IndexedDocsDatabase>) -> Result<()> {

crates/extension/src/wasm_host/wit.rs

@@ -291,6 +291,19 @@ impl Extension {
         }
     }
 
+    pub async fn call_suggest_docs_packages(
+        &self,
+        store: &mut Store<WasmState>,
+        provider: &str,
+    ) -> Result<Result<Vec<String>, String>> {
+        match self {
+            Extension::V010(ext) => ext.call_suggest_docs_packages(store, provider).await,
+            Extension::V001(_) | Extension::V004(_) | Extension::V006(_) => Err(anyhow!(
+                "`suggest_docs_packages` not available prior to v0.1.0"
+            )),
+        }
+    }
+
     pub async fn call_index_docs(
         &self,
         store: &mut Store<WasmState>,