Upgrading to Vue 3.0 and introducing other enhanced checkout forms using other e-commerce services

[john-raymon]

https://v3.vuejs.org/guide/migration/introduction.html#breaking-changes

There's a lot of .sync usage in the codebase that'll need to be modified.

• BREAKING: v-bind's .sync modifier and component model option are removed and replaced with an argument on v-model;