Add system-ui to $font-stack-system

It appears that the working groups have landed on the `system-ui` value
for rendering text using the default user interface font on the platform
on which the UA is running.

Current browser support:
https://caniuse.com/#feat=font-family-system-ui

core/bourbon/library/_font-stacks.scss

@@ -71,8 +71,6 @@ $font-stack-verdana: (
 
 /// A variable that outputs a system font stack.
 ///
-/// @link https://goo.gl/LHRZIf
-///
 /// @type list
 ///
 /// @example scss
@@ -82,10 +80,11 @@ $font-stack-verdana: (
 ///
 ///   // CSS Output
 ///   .element {
-///     font-family: -apple-system, BlinkMacSystemFont, "Avenir Next", "Avenir", "Segoe UI", "Lucida Grande", "Helvetica Neue", "Helvetica", "Fira Sans", "Roboto", "Noto", "Droid Sans", "Cantarell", "Oxygen", "Ubuntu", "Franklin Gothic Medium", "Century Gothic", "Liberation Sans", sans-serif;
+///     font-family: system-ui, -apple-system, BlinkMacSystemFont, "Avenir Next", "Avenir", "Segoe UI", "Lucida Grande", "Helvetica Neue", "Helvetica", "Fira Sans", "Roboto", "Noto", "Droid Sans", "Cantarell", "Oxygen", "Ubuntu", "Franklin Gothic Medium", "Century Gothic", "Liberation Sans", sans-serif;
 ///   }
 
 $font-stack-system: (
+  system-ui,
   -apple-system,
   BlinkMacSystemFont,
   "Avenir Next",

spec/bourbon/library/font_stacks_spec.rb

@@ -21,10 +21,10 @@
                     '"Lucida Typewriter", monospace'
       monaco = '"Monaco", "Consolas", "Lucida Console", monospace'
 
-      system = '-apple-system, BlinkMacSystemFont, "Avenir Next", "Avenir", ' +
-               '"Segoe UI", "Lucida Grande", "Helvetica Neue", "Helvetica", ' +
-               '"Fira Sans", "Roboto", "Noto", "Droid Sans", "Cantarell", ' +
-               '"Oxygen", "Ubuntu", "Franklin Gothic Medium", ' +
+      system = 'system-ui, -apple-system, BlinkMacSystemFont, "Avenir Next", ' +
+               '"Avenir", "Segoe UI", "Lucida Grande", "Helvetica Neue", ' +
+               '"Helvetica", "Fira Sans", "Roboto", "Noto", "Droid Sans", ' +
+               '"Cantarell", "Oxygen", "Ubuntu", "Franklin Gothic Medium", ' +
                '"Century Gothic", "Liberation Sans", sans-serif'
 
       expect(".helvetica").to have_value(helvetica)