[Feature] Introduce .json asset support in theme app extensions

[lukeh-shopify]

WHY are these changes introduced?

Fixes #1962

WHAT is this pull request doing?

Adds .json asset support to theme app extensions. Another PR has already been shipped to enable this upstream

How to test your changes?

• Create a new app with a theme app extension
• Add a .json file in the assets folder
• Run the app dev command
• Notice the theme app extension is pushed as expected

Post-release steps

None

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
• I've made sure that any changes to dev or deploy have been reflected in the internal flowchart.

[github-actions]

Thanks for your contribution!

Depending on what you are working on, you may want to request a review from a Shopify team:

• Themes: @shopify/advanced-edits
• UI extensions: @shopify/ui-extensions-cli
  • Checkout UI extensions: @shopify/checkout-ui-extensions-api-stewardship
• Hydrogen: @shopify/hydrogen
• Other: @shopify/app-management

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	71.77% (+0.01% 🔼)	6942/9673
🟡	Branches	68.71% (+0.02% 🔼)	3419/4976
🟡	Functions	71.22%	1858/2609
🟡	Lines	73.07% (+0.01% 🔼)	6543/8955

Test suite run success

1633 tests passing in 763 suites.

Report generated by 🧪jest coverage report action from 0ff96c7

[jamesmengo]

LGTM but will open this up for another reviewer since I haven't touched these before

[karreiro]

Thank you, @lukeh-shopify! Great stuff 🔥