Don't exclude static local classes

[Marcono1234]

Java 16 added Records, which can be declared locally but are always static. It also allows interfaces to be declared locally.

This pull request makes makes Excluder allow such static local classes. Despite Gson not having a built-in adapter for Record types yet, it at least makes it easier for users to write their own, see #1794 (comment).
The reason why local classes were previously not permitted seems to be that they can capture enclosing references which cannot be serialized and deserialized properly, see #298. However for static classes this won't be an issue.

I have not added a test for this because Gson currently uses Java 6 to compile, so it would require adjustments to the Maven build to support compiling and running one test with Java 16. With Maven this is slightly more cumbersome than with Gradle because users need to manually set up a toolchain to specify a Java 16 JDK, whereas Gradle can handle that on its own.
But you can test this manually by running:

public void test() {
  record R(int i) {}
  record C(R r) {}
  // Without the change this prints "null"
  System.out.println(new Gson().toJson(new C(new R(1))));
}

Relates to #1510

[eamonnmcmanus]

I am wondering if it makes sense for isAnonymousOrLocal to become isAnonymousOrLocalAndNotStatic. In the other case where we call the method (excludeField), would we want to accept static local classes as well?

[Marcono1234]

Ah, you are right excludeField should not exclude fields with static local classes as type either.
Moving it into isAnonymousOrLocal sounds good. What about isAnonymousOrNonStaticLocal as name instead?

[eamonnmcmanus]

Sure, sounds good.

[Marcono1234]

Have force-pushed the change. It now in theory also allows static anonymous classes (to not make the boolean expression too nested), but I don't think such a thing is possible in Java at the moment.