A selector with :extend is never a mixin. fix #67

Author: Horcrux7

src/com/inet/lib/less/LessParser.java

@@ -1,7 +1,7 @@
 /**
  * MIT License (MIT)
  *
- * Copyright (c) 2014 - 2019 Volker Berlin
+ * Copyright (c) 2014 - 2020 Volker Berlin
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -234,7 +234,7 @@ private void parseSemicolon( FormattableContainer currentRule ) {
                     }
                     break;
                 case '(':
-                    if( !reader.nextIsMixinParam( false ) ) {
+                    if( !reader.nextIsMixinParam( false ) || builder.indexOf( ":extend" ) >= 0 ) {
                         builder.append( ch );
                         break;
                     }

test/com/inet/lib/less/samples/less_org_doc/extends/element.css

@@ -0,0 +1,4 @@
+h1,
+h2 {
+  font-size: 20px;
+}

test/com/inet/lib/less/samples/less_org_doc/extends/element.less

@@ -0,0 +1,6 @@
+h1 {
+    font-size: 20px;
+}
+h2 {
+    &:extend(h1);
+}