Pub integration

This change runs "pub get" when a project is created; which will fetch any packages (similar to NuGet Package Restore).

This will fix any errors/warnings in default templates caused by references to pub packages.
I noticed there's a CreateProjectAsync method; but this API is all wonky; I really don't understand how to change it. This *seems* to work ok, despite blocking (VS shows a progress bar, so presumably it's been offloaded to a BG thread).