[css] support IE10 repeat syntax

<!-- Please search existing issues to avoid creating duplicates. --> <!-- Also please test using the latest insiders build to make sure your issue has not already been fixed: https://code.visualstudio.com/insiders/ --> <!-- Use Help > Report Issue to prefill these. --> - VSCode Version: 1.23.1 - OS Version: Windows 8 Steps to Reproduce: 1. View the following code in IE; https://s.codepen.io/fizzbuzz791/debug/EGBxdq/XBkGRZQpnRNA 2. Confirm that it's valid syntax and displays a grid with 3 columns 3. Open a CSS file in VS Code, copy the CSS from codepen and view the following error;  <!-- Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: Yes