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/language/src/language_registry.rs

@@ -746,7 +746,10 @@ impl LanguageRegistry {
                     let capabilities = adapter
                         .as_fake()
                         .map(|fake_adapter| fake_adapter.capabilities.clone())
-                        .unwrap_or_default();
+                        .unwrap_or_else(|| lsp::ServerCapabilities {
+                            completion_provider: Some(Default::default()),
+                            ..Default::default()
+                        });
 
                     let (server, mut fake_server) = lsp::FakeLanguageServer::new(
                         server_id,