[css mode] Add system-ui as value keyword

Closes codemirror#4609

Author: marijnh

mode/css/css.js

@@ -667,7 +667,7 @@ CodeMirror.defineMode("css", function(config, parserConfig) {
     "small", "small-caps", "small-caption", "smaller", "soft-light", "solid", "somali",
     "source-atop", "source-in", "source-out", "source-over", "space", "space-around", "space-between", "spell-out", "square",
     "square-button", "start", "static", "status-bar", "stretch", "stroke", "sub",
-    "subpixel-antialiased", "super", "sw-resize", "symbolic", "symbols", "table",
+    "subpixel-antialiased", "super", "sw-resize", "symbolic", "symbols", "system-ui", "table",
     "table-caption", "table-cell", "table-column", "table-column-group",
     "table-footer-group", "table-header-group", "table-row", "table-row-group",
     "tamil",