Follow-up items to improve #57200.

• Allow the caret to be placed on the # or nullable tokens (currently it must be placed on the enable token)

  roslyn/src/Features/CSharp/Portable/CodeRefactorings/EnableNullable/EnableNullableCodeRefactoringProvider.cs

  Lines 49 to 50 in 1073bba

  	if (!token.IsKind(SyntaxKind.EnableKeyword) || !token.Parent.IsKind(SyntaxKind.NullableDirectiveTrivia))
  	return;

• Allow the refactoring to be triggered from a #nullable restore line

• Allow the refactoring to be triggered from a #nullable disable line

• Improve the light bulb preview associated with the change

• Improve the application preview that appears if the user selects "Preview changes"