add text-color_success utility class

Author: Dottenpixel

ui/utilities/text/_index.scss

@@ -146,6 +146,16 @@
   color: $color-text-error;
 }
 
+/**
+ * @summary Success color of text
+ *
+ * @selector .slds-text-color_success
+ * @modifier
+ */
+.slds-text-color_success {
+  color: $color-text-success;
+}
+
 /**
  * @summary Default color of text on inversed background
  *