Missing syntax highlighting for Augmentations related syntax

[brainwo]

https://github.com/dart-lang/language/blob/main/working/augmentation-libraries/feature-specification.md
https://github.com/dart-lang/dart-syntax-highlight/blob/master/test/test_files/augmentation.dart
https://github.com/dart-lang/dart-syntax-highlight/blob/master/test/test_files/augmented.dart