Add dart-import features

[JCKodel]

I use this plugin along with https://github.com/bluefireteam/vscode-dart-import, but dart-import is broken (i.e.: it leaves the file as not saved when it runs on save).

Since dart-import seems abandoned, it would be a nice feature to get its work and implement it on dart-import-sorter, so, when enabled in options, absolute imports get converted into relative ones.