New Twiddle

controllers.application\.js

import Controller from '@ember/controller';
import { action, computed } from '@ember/object';
import { tracked } from '@glimmer/tracking';

export default class ApplicationController extends Controller {
	@tracked firstName = 'David';
	@tracked lastName = 'Tang';

	// no need for computed decorator
	// @computed('firstName', 'lastName')
	get fullName() {
  	return `${this.firstName} ${this.lastName}`;
  }

	@action
	updateFullName() {
    this.firstName = 'Kevin';
    this.lastName = 'Stenerson';
    
    // don't need set, although this would work too
  	// this.setProperties({
  	// firstName: 'Kevin',
  	// lastName: 'Stenerson'
  	// });
  }
}

templates.application\.hbs

{{fullName}}
<button {{on "click" this.updateFullName}}>Click me</button>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}