Move enum and JsonElement adapter classes to separate class files

[Marcono1234]

Purpose

Make internal code easier to read and maintain by moving some nested classes inside TypeAdapters to separate class files.

Description

Their implementation is complex enough to probably justify having them in their own class files, to make maintaining them easier. For the users this should not be noticeable since these classes are still in the Gson internal package.

I have also added a few comments to the implementations.

Checklist

• New code follows the Google Java Style Guide
  This is automatically checked by mvn verify, but can also be checked on its own using mvn spotless:check.
  Style violations can be fixed using mvn spotless:apply; this can be done in a separate commit to verify that it did not cause undesired changes.
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[eamonnmcmanus]

I like this a lot. I'm certainly not a fan of giant anonymous classes! However, even though people shouldn't be accessing the .internal. APIs, you just know they do. Here is an example. It doesn't cost much to keep the public fields in TypeAdapters and just change how they get their values. So I think we should do that.

[Marcono1234]

Good point, I forgot to consider that. I added back JSON_ELEMENT and ENUM_FACTORY, but not JSON_ELEMENT_FACTORY because it seems to be rarely used (most of these seems to be forks, or decompiled apps). Is that ok?

[eamonnmcmanus]

I think we might as well just keep all the public fields? It doesn't cost much.

[Marcono1234]

Ok, I have added JSON_ELEMENT_FACTORY back as well. I was a bit worried that we keep accumulating @Deprecated fields and methods in Gson's internal code this way, but so far it is not that bad yet.