* Add language: dart
The setup requires that dart be in the users path, such as:
```
export PATH="$HOME/Android/flutter/bin/cache/dart-sdk/bin/:$PATH"
```
Refactor the dart highlights
* lang: dart: add indents and locals
* lang: dart: corrections to local scope
Co-authored-by: Blaž Hrastnik <blaz@mxxn.io>
