fix(core): support underscores in group names

Author: Mirkoddd

sift-core/src/main/java/com/mirkoddd/sift/core/GroupName.java

@@ -25,7 +25,7 @@
  * ensuring that any instantiated GroupName perfectly complies with regex syntax rules.
  */
 final class GroupName {
-    private static final String VALID_NAME_PATTERN = "^[a-zA-Z][a-zA-Z0-9]*$";
+    private static final String VALID_NAME_PATTERN = "^[a-zA-Z][a-zA-Z0-9_]*$";
 
     private static final Pattern VALIDATOR = Pattern.compile(VALID_NAME_PATTERN);