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

examples/src/main/java/com/squareup/moshi/recipes/FallbackEnum.java

@@ -79,7 +79,10 @@ final class FallbackEnum {
         for (int i = 0; i < constants.length; i++) {
           T constant = constants[i];
           Json annotation = enumType.getField(constant.name()).getAnnotation(Json.class);
-          String name = annotation != null ? annotation.name() : constant.name();
+          String name =
+              annotation != null && !Json.UNSET_NAME.equals(annotation.name())
+                  ? annotation.name()
+                  : constant.name();
           nameStrings[i] = name;
         }
         options = JsonReader.Options.of(nameStrings);