Improve VS Code Dart support

[JamesYeoman]

I understand that VS Code isn't where Gapstyle originated from, and so it's not going to be as polished as the Intellij extension. However, the differences are so large. Here's screenshots of main.dart from the flutter hello world app

Intellij

VS Code

[gaplo917]

@JamesYeoman Thank you for using GapStyle.

I haven't support Dart language in VSCode yet. Will take a look in the coming weekend!