Filter completion client-side even if the LSP returns incomplete results

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> Using [rust-analyzer](https://github.com/rust-lang/rust-analyzer), the LSP server could return partial results for completions, where typing additional characters could cause more completions to show that were not visible in the first batch. Currently, the completion popup looks like this: https://user-images.githubusercontent.com/24739403/200195434-a77e619b-ed20-41a2-959d-fe88b44fcf7f.mov There would be a discrepancy between the user's expectations and the actual completions shown, since the filter operation is noticeably delayed. If the completion results were not marked as incomplete, it would look like this: https://user-images.githubusercontent.com/24739403/200195443-1b9c5c57-9e68-4eb9-9d86-39a4e2f7e332.mov The client would filter the results directly. Ideally, the client would filter the results directly even if the server returns an incomplete set, while showing a loading indicator. Then, once the server responds with the next batch of completions after server-side filtering is complete, the completion results would be updated with the final value. This would behave similar to IntelliJ's completion display.