add new link field type

A new field type has been created in the DT core called the Link field type.

It is has been developed, but might have some more tweaks made to it, so this doesn't need to be worked on just yet.

c.f. https://developers.disciple.tools/theme-core/api-posts/post-types-fields-format#links

Start by reading the linked DT core API documentation for the Link field type, then inspect how existing field types are handled in this React Native app. Confirm the field's final format and expected behavior before deciding what implementation and validation are needed; the issue notes that the field may still change.

Written by the indexing model from the issue text.