update for dart sass

[stacyk]

This one is a big one, and I haven't yet spoken with Evan yet to make sure we would all be on board with this type of change, but I am happy to walk everyone through this at the upcoming meeting.

Summary of changes:

• simplified folder structure, less nesting of tiny partials (but of course I made some decisions about where things should go/what the names would be)
• the previous "variables" and "mixin" files are now in the config folder, and are separated out by their purpose/design token they are defining.
• made sure all sass files were using the new module syntax of @use and @forward instead of @import
• The variables are still defined as Sass variables but I included some stuff to automatically convert them to CSS custom properties. You can still use the Sass Variables if you please, but I find it much easier to use their custom property equivalent as you A. don't have to @use the name of the file that variable was created in, and B. custom properties are awesome and more flexible than sass variables.
• Added the .site class in the markup to match the CSS class
• I included a initial/_child-theme.scss file which contains the theme info incase we wanted to compile to the theme folder instead of in assets/css ... but maybe there is another reason to load both CSS files? The gulpfile paths would need to be updated if we want to go that route.

Theres more that I think could still be done to make this a cleaner set of sass files to start with, but I didn't want to re-write too much of the sass. There is likely a number of classes that are not being used still... I didn't do all clean up. Just a light re-org and dart-sass update.