feat: Update Checkstyle rules to allow for shortened SPDX license header (#12)

Author: skaldarnar

checkstyle/checkstyle.xml

@@ -39,7 +39,7 @@
     <!-- Checks that a source file begins with a specified header. --> 
     <module name="RegexpHeader">
         <property name="severity" value="warning"/>
-        <property name="header" value="/*\n * Copyright 20\d\d MovingBlocks\n *\n * Licensed under the Apache License, Version 2.0"/>
+        <property name="header" value="(/*\n * Copyright 20\d\d MovingBlocks\n *\n * Licensed under the Apache License, Version 2.0)|(// Copyright 20\d\d The Terasology Foundation\n// SPDX-License-Identifier: Apache-2.0)"/>
         <property name="fileExtensions" value="java"/>
     </module>