Dart-sass

* **Potentially breaking bug fix:** Members loaded through a nested `import`
are no longer ever accessible outside that nested context.

* Don't throw an error when importing two modules that both forward members with
the same name. The latter name now takes precedence over the former, as per
the specification.

Dart API

* `SassFormatException` now implements `SourceSpanFormatException` (and thus
`FormatException`).

* No user-visible changes

* No user-visible changes.

* Support inline comments in the indented syntax.

* When an overloaded function receives the wrong number of arguments, guess
which overload the user actually meant to invoke, and display the invalid
argument error for that overload.

* When `error` is used in a function or mixin, print the call site rather than
the location of the `error` itself to better match the behavior of calling a
built-in function that throws an error.

Command-Line Interface

* Fix a bug where `--watch` wouldn't watch files referred to by `forward`
rules.

* Fix a bug where selectors were being trimmed over-eagerly when `extend`
crossed module boundaries.