Add example classes to demonstrate usage of Kotlin code from Java and vice versa

wot-dijkstra/src/main/java/org/pgpainless/kotlin_experiments/AJavaClass.java

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+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.kotlin_experiments;
+
+public class AJavaClass {
+
+    /**
+     * Some method in a Java class.
+     * Used by {@link org.pgpainless.kotlin_experiments.java_from_kotlin.AccessJavaFromKotlin}.
+     */
+    public void doSomething() {
+        System.out.println("Something has been done.");
+    }
+}

wot-dijkstra/src/main/java/org/pgpainless/kotlin_experiments/kotlin_from_java/AccessKotlinFromJava.java

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+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.kotlin_experiments.kotlin_from_java;
+
+import org.pgpainless.kotlin_experiments.AKotlinClass;
+
+/**
+ * Example class to verify that we can access Kotlin classes from our Java code \o/.
+ * Accesses {@link AKotlinClass} and executes {@link AKotlinClass#printToStdout()}.
+ */
+public class AccessKotlinFromJava {
+
+    public static void main(String[] args) {
+        // from src/kotlin/.../
+        AKotlinClass kotlin = new AKotlinClass();
+        kotlin.printToStdout();
+    }
+}

wot-dijkstra/src/main/kotlin/org/pgpainless/kotlin_experiments/AKotlinClass.kt

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+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.kotlin_experiments
+
+/**
+ * Some Kotlin class that is used by AccessKotlinFromJava in src/main/java/...
+ */
+class AKotlinClass {
+
+    fun printToStdout() {
+        println("'Hello, World' from Kotlin!")
+    }
+}

wot-dijkstra/src/main/kotlin/org/pgpainless/kotlin_experiments/java_from_kotlin/AccessJavaFromKotlin.kt

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+// SPDX-FileCopyrightText: 2023 Paul Schaub <vanitasvitae@fsfe.org>
+//
+// SPDX-License-Identifier: Apache-2.0
+
+package org.pgpainless.kotlin_experiments.java_from_kotlin
+
+import org.pgpainless.kotlin_experiments.AJavaClass
+
+/**
+ * Verify that we can access Java code from the Kotlin codebase.
+ * Accesses [AJavaClass] and executes [AJavaClass.doSomething].
+ */
+fun main(args: Array<String>) {
+    AccessJavaFromKotlin().run()
+}
+
+class AccessJavaFromKotlin {
+
+    fun run() {
+        val java = AJavaClass()
+        java.doSomething()
+    }
+}