Set default formatter for Dart explicitly #4050

The install instructions for some other formatters (for ex. Prettier) encourage setting it as the global default formatter:

"editor.defaultFormatter": "esbenp.prettier-vscode",

Prettier cannot format Dart files so trying to format produces fails with a message. We can set the default for Dart explicitly so that it will be used first. Users can still override this by setting a default formatter in the "[dart]" section of their configuration.