Dart-sass

* Allow the null safety release of stream_transform.

* Allow `forward...with` to take arguments that have a `!default` flag without
a trailing comma.

* Improve the performance of unitless and single-unit numbers.

Node JS API

* Fix Electron support when `nodeIntegration` is disabled.

Dart API

* All range checks for `SassColor` constructors now throw `RangeError`s with
`start` and `end` set.

* **Potentially breaking bug fix:** When using `for` with numbers that have
units, the iteration variable now matches the unit of the initial number. This
matches the behavior of Ruby Sass and LibSass.

Node JS API

* Fix a few infrequent errors when calling `render()` with `fiber` multiple
times simultaneously.

* Avoid possible mangled error messages when custom functions or importers throw
unexpected exceptions.

* Fix Electron support when `nodeIntegration` is disabled.

* No user-visible changes.

* Optimize `==` for numbers that have different units.

* Print the actual number that was received in unit deprecation warnings for
color functions.