Update preview language features for start of JDK 13

With JDK 13 development commencing soon, a determination needs to be made on whether or not the preview language features from JDK 12 (expression switch and raw string literals) should continue as preview features, graduate to full features, or be removed.

At least for the start of JDK 13, to gather more feedback, continuing both of these features as preview features seems prudent.

Later in JDK 13 release, a subsequent determination can be made to continue as preview, graduate, or remove.