Dart-sass

* Use an updated version of `node_preamble` when compiling to JS.

* Fixes an error when emitting source maps to stdout.

* No user-visible changes.

* Fix a bug where escape sequences were improperly recognized in `else` rules.

JavaScript API

* Add `sass.NULL`, `sass.TRUE`, and `sass.FALSE` constants to match Node Sass's
API.

* If a custom Node importer returns both `file` and `contents`, don't attempt to
read the `file`. Instead, use the `contents` provided by the importer, with
`file` as the canonical url.

* No user-visible changes.

* Be more memory-efficient when handling `forward`s through `import`s.