Add `related_applications` and `prefer_related_applications` to WPT

[aarongustafson]

Moving these two items out of #666. They should be added to WPT once there is additional uptake of the properties, but we do not want them to hold up CR.

• related_applications (currently single-engine, Related applications members (at risk?) #877)
• prefer_related_applications (currently single-engine, Related applications members (at risk?) #877)

[marcoscaceres]

We probably want to move this issue to WPT instead.