Don't escape non-snippets in assist
I was misunderstanding that we're always sending snippets as response to assist request. For assists that never return snippets like `move_const_to_impl` we don't need to escape, and I don't think `utils::escape_non_snippet()` is useful at the moment since we guarantee that only a single edit will have `InsertTextFormat.Snippet` and we have `utils::render_snippet()` for that.
c552e5a55f