Migrating theme commands over to CLI 3.0

Support for themes has been added to Shopify CLI 3.20. In addition to migrating over theme commands, Shopify CLI 3.20 now includes the ability to integrate Shopify CLI into your CI/CD pipeline to perform actions like pushing, pulling and publishing a theme. Most theme commands on Shopify CLI 3.20 will continue to work the same with the exception of a few differences outlined here. More information on using Shopify CLI in a CI/CD pipeline can be found here.