Dart-sass

* No user-visible changes.

* No user-visible changes.

* No user-visible changes.

* No user-visible changes.

Dart API

* **Potentially breaking bug fix:** `SassNumber.asSlash`,
`SassNumber.withSlash()`, and `SassNumber.withoutSlash()` have been marked as
`internal`. They were never intended to be used outside the `sass` package.

* **Potentially breaking bug fix:** `SassException` has been marked as `sealed`
to formally indicate that it's not intended to be extended outside of the
`sass` package.

* Add a `Value.withListContents()` method that returns a new Sass list with the
same list separator and brackets as the current value, interpreted as a list.

Dart API

* Added `compileToResult()`, `compileStringToResult()`,
`compileToResultAsync()`, and `compileStringToResultAsync()` methods. These
are intended to replace the existing `compile*()` methods, which are now
deprecated. Rather than returning a simple string, these return a
`CompileResult` object, which will allow us to add additional information
about the compilation without having to introduce further deprecations.

* Instead of passing a `sourceMaps` callback to `compile*()`, pass
`sourceMaps: true` to `compile*ToResult()` and access
`CompileResult.sourceMap`.

* The `CompileResult` object exposes a `loadedUrls` object which lists the
canonical URLs accessed during a compilation. This information was
previously unavailable except through the JS API.