Add generic system font families

This adds support for the following generic font families, which were
added to the [CSS Fonts Module Level 4][css-fonts-level-4]:

- `system-ui`
- `ui-monospace`
- `ui-rounded`
- `ui-sans-serif`
- `ui-serif`

[css-fonts-level-4]: https://drafts.csswg.org/css-fonts-4/

Author: tysongach

grammars/css.cson

@@ -1481,7 +1481,7 @@
         'name': 'support.constant.vendored.property-value.css'
       }
       {
-        'match': '(?<![\\w-])(?i:arial|century|comic|courier|garamond|georgia|helvetica|impact|lucida|symbol|system|tahoma|times|trebuchet|utopia|verdana|webdings|sans-serif|serif|monospace)(?![\\w-])'
+        'match': '(?<![\\w-])(?i:arial|century|comic|courier|garamond|georgia|helvetica|impact|lucida|symbol|system-ui|system|tahoma|times|trebuchet|ui-monospace|ui-rounded|ui-sans-serif|ui-serif|utopia|verdana|webdings|sans-serif|serif|monospace)(?![\\w-])'
         'name': 'support.constant.font-name.css'
       }
     ]