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;
    +    }