Update to support forthcoming SCSS module system

[smithki]

SCSS is making some major updates to its module system. The changes are proposed to land in Libsass (C/Node bindings) by October 2019. The changes will not be breaking for some time:

One year after both implementations have launched stable support for the full module system proposal (tentatively 1 October 2020), we will deprecate @import as well as all global core library function calls that could be made through modules.

I'm opening this issue as a reminder to support the new syntax by end of year.

Feel free to assign this to me. I am happy to implement the necessary changes.

[smithki]

Just to note: I believe the changes to SCSS core will make the "primary interface" (@include A(...)) and (less certain) @include A(config, ...) obsolete.