Add a library for interpreting application/linkset+json responses

[gabesullice]

The proposed menu endpoint returns JSON formatted using the linkset media type. This library provides an easy way to extract links from such a linkset response.

This does not have any customizations for dealing with menus specifically, as I hope that we can share this library with the broader ecosystem.

The library only exports two concrete functions—denormalize and parse—and a number of interfaces. denormalize takes an plain object and "upcasts" it into an object that adheres to the new LinksetInterface. parse is a simple wrapper around denormalize that takes a JSON string instead of an object. LinksetInterface provides a few helpful utilities for understanding the set of links returned in a linkset response. This can help clients program RESTful behaviors into their UIs.

For example, the following pseudo-code shows how a linkset could be used to automatically prompt a user for login credentials and then start a session:

import { parse } from '@jsdrupal/linkset`;

const linkset = parse(menuJSON);

if (linkset.hasLinkTo('authenticate')) {
  // ... get user credentials ...
  fetch(linkset.linkTo('authenticate').href, {
    method: 'POST',
    body: JSON.stringify(credentials),
    credentials: 'cross-origin',
  });
}