Add @Json.ignore (#1417)

* Default Json.name to an unset value

* Promote shared transient tests to DualKotlinTest

* Add new ignore property to Json

* Support it in ClassJsonAdapter

* Mention no enum/record support

* Support in KotlinJsonAdapter

* Rework code gen API to know of "ignored"

* Support in apt code gen

* Support in KSP

* Update old non-working transient example test

* Synthetic holders

* Use field on both

adapters/src/main/java/com/squareup/moshi/adapters/EnumJsonAdapter.java

@@ -68,7 +68,10 @@ public final class EnumJsonAdapter<T extends Enum<T>> extends JsonAdapter<T> {
       for (int i = 0; i < constants.length; i++) {
         String constantName = constants[i].name();
         Json annotation = enumType.getField(constantName).getAnnotation(Json.class);
-        String name = annotation != null ? annotation.name() : constantName;
+        String name =
+            annotation != null && !Json.UNSET_NAME.equals(annotation.name())
+                ? annotation.name()
+                : constantName;
         nameStrings[i] = name;
       }
       options = JsonReader.Options.of(nameStrings);