Provide mechanism to query preview feature status for annotation processors

Summary

Provide a method on ProcessingEnvironment to allow the whether or not preview features are enabled to be determined.

Problem

When generating code, a sophisticated annotation processor could benefit from knowing whether or not preview language feature are available.

Solution

Provide a predicated on ProcessingEnvironment to return whether or not preview features are enabled.

Specification

+    /**
+     * Returns {@code true} if <em>preview features</em> are enabled
+     * and {@code false} otherwise.
+     * @return whether or not preview features are enabled
+     *
+     * @implSpec The default implementation of this method returns
+     * {@code false}.
+     *
+     * @since 13
+     */
+    default boolean isPreviewEnabled() {
+        return false;
+    }