Backport Node 14 syntax to comply to minimum reqs

[nikhiltri]

This PR fixes a bug while running twill:build in a Node v8 environment.

While updating our site to the latest version of Twill 2—2.13.0, we ran into a bug while running twill:build. I have all the composer packages and their dependencies updated and installed and I ran php artisan twill:update successfully. But I'm running into a bug when I run php artisan twill:build:

 ERROR  Error loading /home/vagrant/website/vendor/area17/twill/vue.config.js:
 ERROR  SyntaxError: Unexpected token ?
/home/vagrant/website/vendor/area17/twill/vue.config.js:169

The culprit is this line here that uses the nullish coalescing operator (??):

const host = process.env.APP_URL.split('//')[1] ?? process.env.APP_URL;

I'm running node v8.17.0 and npm v6.13.4, as the Twill documentation states that npm 6.13 is the recommended version. But according to node.green the nullish coalescing operator was introduced in node v14

This PR refactors vue.config.js to not use the ?? operator.