Known types - LI element limited to multiple types with at least 1 item

[Sevitas]

Motivation

Solves scenario 4 of issue #90 scenarios
Linked items element limited to multiple types with at least 1 item.

Proposed solution

Article.Generated.cs will contain

public IEnumerable<IContentItem> Sections { get; set; }

Article.Typed.Generated.cs will contain

public IEnumerable<Banner> SectionsBannerTyped => Sections.OfType<Banner>();
public IEnumerable<CompanyNews> SectionsCompanyNewsTyped => Sections.OfType<CompanyNews>();

Additional context

As a developer, you still need to check which properties contain any items. Some properties might be empty.
Take in mind responses and notes in the comment.