Make StylingPolicy public

Did not use a builder since having access to StylingPolicy.apply can be very critical

Author: EugenMayer

owasp-java-html-sanitizer/src/main/java/org/owasp/html/StylingPolicy.java

@@ -42,7 +42,7 @@
  * ones to reduce the attack-surface.
  */
 @TCB
-final class StylingPolicy implements JoinableAttributePolicy {
+public final class StylingPolicy implements JoinableAttributePolicy {
 
   final CssSchema cssSchema;
   final Function<String, String> urlRewriter;