Library for testing types: expect-type (what ember-source uses).

Example:

• https://github.com/NullVoxPopuli/ember-resources/tree/main/ember-resources
• https://github.com/emberjs/ember.js/

Will require that we update the eslint configs (I had to do that for my own stuff here)

Open questions:

• ember.js has a separate tsconfig.json for the type tests -- do we need this? I did not find that I needed it for ember-resources

  answer: we do need this -- ember-resources' types are generated through rollup, rather than tsc/glint