Add debug logging for loading .env files

[t-kelly]

WHY are these changes introduced?

I just spent 2+ hours trying to debug why I was receiving a 401 when running shopify theme dev. I understood from the debug logging that my Admin API session was invalid and https://theme-kit-access.shopifyapps.com/cli/admin/api/unstable/themes.json was failing, but even after running shopify auth logout I was not being prompted to login via browser (as I normally do).

Finally after adding some additional logging, I found that $SHOPIFY_CLI_THEME_TOKEN was being set. I had forgotten that I had created a .env file several weeks ago and it was setting the --password flag in this case

WHAT is this pull request doing?

Just knowing that a .env file is being loaded would have probably saved me a few hours of debugging. One-liner log to let people know that an .env file was found and being loaded.

Measuring impact

How do we know this change was effective? Please choose one:

• [x] n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• [ ] Existing analytics will cater for this addition
• [ ] PR includes analytics changes to measure impact

Checklist

• [x] I've considered possible cross-platform impacts (Mac, Linux, Windows)
• [x] I've considered possible documentation changes

[t-kelly]

CLA signed -- can you please re-run the status check?

[github-actions[bot]]

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[t-kelly]

This is still relevant -- let's get it merged team!