Switch from LibSassBuilder to DartSassBuilder

[rockfordlhotka]

Apparently LibSassBuilder is no longer the good way to build scss in a Blazor project, and DartSassBuilder is better.

• https://github.com/johan-v-r/LibSassBuilder/issues/42
• https://www.nuget.org/packages/DartSassBuilder/

Thanks to @allenconway for the info!