Use modern dart-sass API instead of deprecated legacy API

Dart Sass began emitting warnings about using the legacy API in version 1.79.0 (released about 3 weeks ago, see https://github.com/sass/dart-sass/pull/2343), so it'd be worth migrating to the [modern API](https://sass-lang.com/documentation/js-api/#md:usage) soon as postcss-sass is currently still [using the legacy API](https://github.com/csstools/postcss-sass/blob/main/src/index.mjs#L84).