add helpful hints in error message if unknown decorator

[iscai-msft]

I think it would be helpful when you get "Unknown identifier" / "Unknown decorator" errors to include a line like "make sure you're using the libraries you need to use" or something along those lines, to point people to either adding a using statement, or fully qualifying their identifier / decorator

[timotheeguerin]

Wonder if the solution here is just adding the quick fix option to resolve it for you. Typescript doesn't really give you more info but they do have the quick fix

[iscai-msft]

oh yeah that's even better!