add sass_api to supported package list

[Goodwine]

The csslib package used in DartPad is unmaintained. I propose replacing it with the actively supported sass and sass_api packages. These alternatives provide modern CSS parsing with the additional benefit of supporting Sass.

[parlough]

Thanks for opening an issue :D

DartPad doesn't support use of package:csslib, at least not intentionally, although it might be a transitive dependency. Did you see somewhere that says it's supported or where it is used?

As for the sass packages, do you have any particular use in mind for them (in the context of DartPad being a playground)?