add react import for public extension build

[faouzioudouh]

This is just an improvement PR Related to Shopify/shopify-cli-extensions#127, #577, #588 and #587

Conditionally remove react import from checkout-ui react template when using node build (isShopify)

WHAT is this pull request doing?

How to test your changes?

Public build:

• Clone this branch
• Generate new app
• Geneate new extension using yarn shopify app generate extension --path path-to-app
• you should see react import at the top of src/index.(t/j)sx file

Shopify build:

• Clone this branch
• Generate new app
• Geneate new extension using SHOPIFY_RUN_AS_USER=1 yarn shopify app generate extension --path path-to-app
• react import should no exist at the top of src/index.(t/j)sx file

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.

[isaacroldan]

This is a temporary fix while both Node and Go servers live together right? Is this fix urgent?
In a week or so we'll make the Node server the default, so this code will have a short life... can we wait until that moment?

[faouzioudouh]

@isaacroldan yes this is a temporary fix and also not urgent, if Node server will be the default in a week let's not merge this PR then, as it won't be needed

Thanks for the feedback

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