Add --silence-deprecation Dart Sass options (slice?)

[razonyang]

I would like to request a feature to silence deprecation warnings from Dart Sass.

{{ $css := resources.Get "index.scss" | toCSS (dict
  "transpiler" "dartsass"
  "silenceDeprecation" "import"
) }}

Reference: https://sass-lang.com/blog/import-is-deprecated/#controlling-deprecation-warnings

[bep]

Note that these options must be passed via the protobuf API (https://github.com/bep/godartsass); I assume they are also available there and not just as CLI flags.