[folding] highlight folding range when hovering over the collapse button

<!-- ⚠️⚠️ 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. --> In other editors like Visual Studio the region you are about to collapse gets highlighted when you are hovering over the "-" button to collapse it:  VS code does not have a way to do this as far as I can tell.