red_knot_server: add auto-completion MVP

This PR does the wiring necessary to respond to completion requests from
LSP clients.

As far as the actual completion results go, they are nearly about the
dumbest and simplest thing we can do: we simply return a de-duplicated
list of all identifiers from the current module.

crates/red_knot_server/src/server.rs

@@ -227,6 +227,9 @@ impl Server {
             inlay_hint_provider: Some(lsp_types::OneOf::Right(
                 InlayHintServerCapabilities::Options(InlayHintOptions::default()),
             )),
+            completion_provider: Some(lsp_types::CompletionOptions {
+                ..Default::default()
+            }),
             ..Default::default()
         }
     }