feat: modular-css feature syntax test CSS file

Author: AndrewLeedham

tests/features.css

@@ -0,0 +1,20 @@
+@value male: #0000FF;
+@value female: pink; 
+@value one from "all";
+@value one, two from "all";
+@value * as namespace from "all";
+
+.dad{
+    composes: parent from "./family";
+    background: male;
+}
+
+.mum{
+    composes: parent from "family";
+    border: 1em solid female;
+}
+
+:external(child from "family"){
+    composes: dad;
+    composes: mum;
+}