Fix shopify theme package to support theme blocks

[montalvomiguelo]

WHY are these changes introduced?

Fixes #5135

WHAT is this pull request doing?

Updates shopify theme package to support theme blocks.

How to test your changes?

• Download https://github.com/Shopify/reference-theme
• Run shopify theme package --path ~/Downloads/reference-theme
• Notice the zip file includes theme blocks

Post-release steps

Measuring impact

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

• 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

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

[aswamy]

Worked as described. Thanks for the fix 👍 🎩

[jamesmengo]

Hey @montalvomiguelo!
Thanks for these changes, it's awesome seeing the community come together to help each other out.

I'm hoping to roll this out in tomorrow's release, so I created a duplicate PR here with your changes + attribution so that I could address any CI failures. Let's use that PR instead for this change.

Thank you so much for your contribution, it's greatly appreciated!