LUCENE-9694: make new DocumentSelector interface public so it is usable outside of its own package

lucene/misc/src/java/org/apache/lucene/misc/index/IndexRearranger.java

@@ -130,7 +130,7 @@ public class IndexRearranger {
   }
 
   /** Select document within a CodecReader */
-  interface DocumentSelector {
+  public interface DocumentSelector {
     BitSet getFilteredLiveDocs(CodecReader reader) throws IOException;
   }
 }