WHY are these changes introduced?

First step for https://github.com/Shopify/shopify-cli-planning/issues/332
Fixes https://github.com/Shopify/shopify-cli-planning/issues/365

A very similar PR was merged and then reverted, as we wanted to finish other related work first.

⚠️ We are still testing this functionality, it will be ready to use for everyone when we properly announce it and publish the documentation.

WHAT is this pull request doing?

• Enable Theme Kit Access passwords through env variable. When SHOPIFY_CLI_ADMIN_AUTH_TOKEN contains a Theme Kit Access password, the theme requests will go through Theme Kit Access API instead of Admin API, so the password is validated. As the login is not required, it also requires passing the store URL through SHOPIFY_SHOP.
• Rename SHOPIFY_CLI_STORE to SHOPIFY_SHOP, which was already existing. It was added recently to be used from the CLI3. I'll update the CLI3 as well to use SHOPIFY_SHOP.

How to test your changes?

Check that theme commands work the same way when logged in (by using the Admin API):

• bin/shopify login
• DEBUG=1 bin/shopify theme list

Check that they also work with a Theme Kit Access password (by using Theme Access Kit API):

• Install Theme Kit Access app and generate a new password
• bin/shopify logout
• DEBUG=1 SHOPIFY_SHOP=your-shop.myshopify.com SHOPIFY_CLI_ADMIN_AUTH_TOKEN=your-password bin/shopify theme list

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above (if needed).