[jnigen] Export and document j_elements.dart

[liamappelbe]

• Export and document the classes in j_elements.dart, so that users can write custom visitors.
  • Make Element private, and delete Classes.let
• This caused export conflicts because we were exporting elements.dart, which has a bunch of AST elements with the same names as the ones in j_elements.dart.
  • elements.dart conceptually should be private. It was exported because Config.importedClasses uses it. So I made Config.importedClasses private.
  • But it (and importClasses()) are needed by other files in JNIgen. So I moved them to an extension method, ConfigInternal, which is hidden when config_types.dart is exported . That way internal uses of Config can see the method, but external users can't.

Fixes #2962

[github-actions]

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