Add support for adding an rdf-type to data that is submitted in an ui:form

Use case

I would like to present my users a "review" form to produce data like:

<#Example1> a <https://schema.org/Review>;
       <https://schema.org/abstract> "The service was fantastic".

In the form they just should be able to fill in the abstract and not be asked to select from a Choice of rdf-types they want to create.

Missing

In the current UI forms I only am permitted to set ui:property-s , which can't be used to set an rdf-type.

@timea-solid suggested to use a ui:Choice, but that adds an extra menu item in the display which the user is forced to set to a particular value, and creates the possibility to add many resources of that type.

Suggestion

A suggestion by @timbl : "add a new type of field which is a constant field, along with the name field say “also add ?x a ExampleObject whenever you set this name”.. have to make sure it gets removed if the name goes too"

Review the linked src/editDataPage/helloWorldForm.ttl example and the existing ui:form, ui:property, and ui:Choice concepts described in the issue. Define how a constant field should add the requested rdf:type without a user choice, how it behaves when the name is removed, and how the form should represent the completed review data.