Show/expand Dart templates in source editor

Dart supports documentation templates, for example:

```dart
/// This is the dart doc for some property.
///
/// {@template some_template}
/// This explanation is needed many places in our docs, so we wrap it with a
/// template.
/// {@endtemplate}

/// This is another doc for some other property, which replicates our templated
/// doc.
///
/// {@macro some_template}
```

This templating behavior is critical to keep related documentation in sync. However, these templates are a significant nuisance for developers who are working in source code.

I routinely jump into Flutter source to read docs, only to find a `@macro` where I was expecting documentation. I have the same problem in my own source code where macros are used.

I'd like to see one or both of the following in the IDE:

- Source linking for template macro's so that I can `CMD+B` to the definition of the template.
- Automatic IDE expansion of a macro template so that I can read a template inline, as if it were written in the source code docs that I'm reading.