feat: add hatch for opt-in experimental types

Signed-off-by: Sam Gammon <sam@elide.dev>

Author: sgammon

pkl-commons/api/pkl-commons.api

@@ -31,3 +31,6 @@ public final class org/pkl/commons/UrisKt {
 	public static final fun toPath (Ljava/net/URI;)Ljava/nio/file/Path;
 }
 
+public abstract interface annotation class org/pkl/commons/annotations/PklExperimental : java/lang/annotation/Annotation {
+}
+

pkl-commons/src/main/kotlin/org/pkl/commons/annotations/PklExperimental.kt

@@ -0,0 +1,25 @@
+/*
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * 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
+ *
+ *     https://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.
+ */
+package org.pkl.commons.annotations
+
+/**
+ * Marks experimental APIs in Kotlin or Java. Such APIs may be publicly exposed by the Pkl project,
+ * but are not provided with a stability guarantee, and require opt-in use from Kotlin.
+ */
+@RequiresOptIn(level = RequiresOptIn.Level.WARNING)
+@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY)
+@Retention(AnnotationRetention.BINARY)
+annotation class PklExperimental