[w3c/manifest] Add `related_applications` and `prefer_related_applications` to WPT (#950)

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, #877)
- [ ] prefer_related_applications (currently single-engine, #877)

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