Add label_for_completion to extension API (#10175)

This PR adds the ability for extensions to implement
`label_for_completion` to customize completions coming back from the
language server.

We've used the Gleam extension as a motivating example, adding
`label_for_completion` support to it.

Release Notes:

- N/A

---------

Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/project/src/project.rs

@@ -9915,6 +9915,8 @@ async fn populate_labels_for_completions(
         lsp_adapter
             .labels_for_completions(&lsp_completions, language)
             .await
+            .log_err()
+            .unwrap_or_default()
     } else {
         Vec::new()
     };