Dart-sass

* **Potentially breaking bug fix:** Null values in `use` and `forward`
configurations no longer override the `!default` variable, matching the
behavior of the equivalent code using `import`.

* Use the proper parameter names in error messages about `string.slice`

* Fix a bug that disallowed more than one module from extending the same
selector from a module if that selector itself extended a selector from
another upstream module.

* Fix a bug where bogus indented syntax errors were reported for lines that
contained only whitespace.

* No user-visible changes.

* Fix a typo in a deprecation warning.

JavaScript API

* Drop support for Chokidar 2.x. This version was incompatible with Node 14, but
due to shortcomings in npm's version resolver sometimes still ended up
installed anyway. Only declaring support for 3.0.0 should ensure compatibility
going forward.

Dart API

* Allow the null safety release of args and watcher.

Command Line Interface

* Add a `-w` shorthand for the `--watch` flag.

* Update chokidar version for Node API tests.

JavaScript API

* Allow a custom function to access the `render()` options object within its
local context, as `this.options`.