feat: add placeholder main class as starting point

packages/core/java/src/main/java/org/itk/wasm/Main.java

@@ -0,0 +1,26 @@
+/*-
+ * #%L
+ * Java bindings for itk-wasm.
+ * %%
+ * Copyright (C) 2023 ITK developers.
+ * %%
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * #L%
+ */
+package org.itk.wasm;
+
+public class Main {
+  public static void main(String... args) {
+    System.out.println("Hello world");
+  }
+}