add detect for variable fonts (#2202)

feature-detects/css/variablefonts.js

@@ -0,0 +1,18 @@
+/*!
+{
+  "name": "Variable Open Type Fonts",
+  "property": ["variablefonts"],
+  "authors": ["Patrick Kettner"],
+  "tags": ["css"],
+  "notes": [{
+    "name": "Variable fonts on the web",
+    "href": "https://webkit.org/blog/7051/variable-fonts-on-the-web/"
+  }, {
+    "name": "Variable fonts for responsive design",
+    "href": "https://alistapart.com/blog/post/variable-fonts-for-responsive-design"
+  }]
+}
+!*/
+define(['Modernizr', 'testAllProps'], function(Modernizr, testAllProps) {
+  Modernizr.addTest('variablefonts', testAllProps('fontVariationSettings'));
+});

lib/config-all.json

@@ -111,6 +111,7 @@
     "css/transitions",
     "css/userselect",
     "css/valid",
+    "css/variablefonts",
     "css/vhunit",
     "css/vmaxunit",
     "css/vminunit",