8344314: Revert removal of jdk.internal.java.PreviewFeature.CLASSFILE_API

Reviewed-by: liach

Author: asotona

src/java.base/share/classes/jdk/internal/javac/PreviewFeature.java

@@ -74,6 +74,7 @@ public enum Feature {
         SCOPED_VALUES,
         @JEP(number=480, title="Structured Concurrency", status="Third Preview")
         STRUCTURED_CONCURRENCY,
+        CLASSFILE_API,
         STREAM_GATHERERS,
         @JEP(number=494, title="Module Import Declarations", status="Second Preview")
         MODULE_IMPORTS,