Allow build 3.0.0, min language 3.8.0, generate new style, reformat (#399)

* Allow build 3.0.0, update for build_test 3.0.0.

* Reformat to 3.8.0 style.

* Generating in latest language style.

Author: davidmorgan

packages/graphql_codegen/example/lib/fragments.graphql.dart

@@ -9,9 +9,7 @@ void main() async {
   );
   final result = await client.query$FetchPerson(
     Options$Query$FetchPerson(
-      variables: Variables$Query$FetchPerson(
-        id: "id1",
-      ),
+      variables: Variables$Query$FetchPerson(id: "id1"),
     ),
   );
   final parased = result.parsedData;