Add `@macro` comment expansion

[feinstein]

Many times we are looking at some widget's source-code and we find something like this:

  /// {@macro flutter.services.TextInputConfiguration.enableIMEPersonalizedLearning}
  final bool enableIMEPersonalizedLearning;

I can go to the variable and hit the shortcut for showing the definition, which in turn will pull the macro text, but I wanted to hit F4 and go to the macro definition itself, so I can read it as code, and not in the small doc pop-up window.