`runPubGetOnPubspecChanges` implementation is outdated

[vkammerer]

In order to address Dart-Code/Dart-Code#4110, runPubGetOnPubspecChanges was changed from a boolean to a always/prompt/never enum.
Dart-Code/Dart-Code@029850e is already merged and deployed to VSCode end users.

It means that the setting generated by this extension is now flagged by the linter:

[blaugold]

Thanks for reporting this.