Implement external and abstract variables

[eernstg]

Cf. the implementation plan, the addition of external and abstract variables to Dart may require some updates.

[srawlins]

Currently dartdoc only indicates an abstract class or an abstract method via very muted text in the title:

• https://pub.dev/documentation/analyzer/0.39.17/dart_element_element/Element-class.html
• https://pub.dev/documentation/analyzer/0.39.17/dart_element_element/Element/accept.html

I don't think "external" methods are indicated in any way. 😕

• https://api.dart.dev/stable/2.9.2/dart-collection/HashMap/HashMap.html

So I think to complete this feature, I will annotate abstract fields in the same way that abstract classes and methods are now. I don't think there is any work to be done for external fields or variables.