Two scenarios:

1. Start using nullability annotations in code that is not nullable-enabled

• we can offer to nullable-enable the project (especially if there's only one file in the project)
• we can offer to nullable-enable the current file

2. From a source-level #nullable enable directive, offer to shift it to the project (as we did for roslyn here):
   This involves toggling directives in all the files in the project:
   • top-level #nullable enable in files gets removed
   • files that were not previously enabled get #nullable disable
   • #nullable restore becomes #nullable disable
   • #nullable enable in the middle of a file becomes #nullable restore