JDK 13 RFR of JDK-8222378: Provide mechanism to query preview feature status for annotation processors

[Joe Darcy]

Hello,

Please review the code changes and CSR to address:

     JDK-8222378: Provide mechanism to query preview feature status for 
annotation processors
     webrev: http://cr.openjdk.java.net/~darcy/8222378.0/
     CSR: https://bugs.openjdk.java.net/browse/JDK-8222424

In brief, a isPreviewEnabled predicate default method is added to 
ProcessingEnvironment to allow processors to query whether or not 
preview features (JEP 12) are enabled.

Thanks,

-Joe