Add aspect-ratio feature (web-platform-dx#602)

Author: foolip

feature-group-definitions/aspect-ratio.yml

@@ -0,0 +1,8 @@
+name: aspect-ratio
+description: "The `aspect-ratio` CSS property controls the width-to-height ratio of elements. For `<img>` and `<video>` elements, the `width` and `height` attributes used together with `height: auto` control the aspect ratio while the image/video is loading."
+spec: https://drafts.csswg.org/css-sizing-4/#aspect-ratio
+usage_stats: https://chromestatus.com/metrics/css/timeline/popularity/657
+compat_features:
+  - css.properties.aspect-ratio
+  - html.elements.img.aspect_ratio_computed_from_attributes
+  - html.elements.video.aspect_ratio_computed_from_attributes