Serializable fields are not working in D10.

Problem/Motivation

In core, REST views work by building a render array as for a normal view, and then passing it through a normalizer and serializer to turn it into JSON instead of HTML. Unfortunately, this limits the emitted JSON data for each field to what would normally be displayed in HTML.

This module's premise is to hook into the field system via field display plugins, and (instead of markup strings or render arrays) return a special object that, when normalized and serialized, returns the desired data.

Unfortunately, in order to do so, the special object must pass unscathed through the render system, which (since it is designed to work with strings and markup) may irreversibly flatten the object into its string representation. One of the new ways in which this can happen in Drupal 10 is by assigning it to a typed property (or returning it from a return-typed function) that is typed "string|MarkupInterface", eg FieldPluginBase::$last_render.

Steps to reproduce

- Create a REST view that uses the Fields display mode, and use any of the "(serializable)" field handlers provided by this module.
- Open the view.

Expected result:

- The JSON output contains the correct data for the field.

Actual result:

- The JSON output contains "[...]".

Proposed resolution

This can (for now) be avoided by "camouflaging" the RenderableData and SerializedData types from this module as MarkupInterface, allowing it to be assigned to properties and returned from functions that specify the "string|MarkupInterface" type.