See #5671 (comment)

Widget Preview does support a Pub workspace, but probably needs invoking at the root of the workspace and not within a single project.

We should support this like the CLI does, even though we don't support multiple unrelated projects.

@bkonyi the only concern I have here, is what if I open a subset of a Pub Workspace. For example if I open just pkg/analysis_server in the SDK (assuming it was Flutter) - would widget preview scan the entire SDK (Pub Workspace) for widget previews?

I wonder if we could restrict this to just the open folders (which perhaps could be done together with flutter/flutter#173550?)

(cc @GabrielRStabile)