Update CLI env variable names

[paulomarg]

Description

With https://github.com/Shopify/cli/pull/1789, the CLI will deprecate some env var names. While they will still be present so previous apps won't break, we should migrate new apps as soon as possible to the new format.

This PR adds support for the new variable names, but falls back to the current ones so it won't affect existing apps.

Checklist:

• [x] My commit message follow the pattern described in here
• [x] I have performed a self-review of my own code.
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added a changelog line.

[paulomarg]

We've decided to close this for now since we might want to take a slightly different approach in the future!