Add text-spacing-trim (web-platform-dx#570)

Author: foolip

feature-group-definitions/text-spacing-trim.yml

@@ -0,0 +1,6 @@
+name: text-spacing-trim
+description: The `text-spacing-trim` CSS property controls spacing around CJK characters, avoiding excessive whitespace when using full-width punctuation characters.
+spec: https://drafts.csswg.org/css-text-4/#text-spacing-trim-property
+usage_stats: https://chromestatus.com/metrics/css/timeline/popularity/771
+compat_features:
+  - css.properties.text-spacing-trim