[Themes] Remove Ruby invocation from the `app dev` command

[jamesmengo]

WHY are these changes introduced?

Closes https://github.com/Shopify/develop-advanced-edits/issues/362

WHAT is this pull request doing?

• Remove legacy flag from app dev command
• Delete theme-app-extension.js (legacy implementation)
• Update imports + tests

How to test your changes?

• Try running app dev with the legacy flag. This should complain about a non-existent flag

Post-release steps

• https://github.com/Shopify/cli/pull/4593
• https://github.com/Shopify/cli/pull/4595

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
• [x] 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