Update SolidusBraintree InstallGenerator to install frontend code

Summary

We want the SolidusBraintree InstallGenerator to install the frontend code so that users can customize it easily.

Closes https://github.com/solidusio/solidus_braintree/issues/104.

Known concern: SolidusBraintree 2.0 is blocked by Solidus 3.4

We've updated SolidusBraintree to be compatible with the edge Solidus and SolidusStarterFrontend versions. As a result, the present SolidusBraintree would require Solidus and SolidusStarterFrontend 3.4.0. Thus, we can only release SolidusBraintree 2.0 once Solidus 3.4.0 is released.

If we want SolidusBraintree 2.0 to be compatible with the latest stable version of Solidus (3.3.1), we can create a separate branch of this repo targetting that Solidus version. We'll then release SolidusBraintree 2.0.0 from that branch.

Testing

I was able to successfully update a Rails app with solidus_paypal_braintree to this new version of SolidusBraintree. See https://github.com/gsmendoza/solidus_store/tree/rails-7-0-4--solidus-3-4-0-dev--braintree--renamed.

Checklist

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[x] I have written a thorough PR description.
[x] I have kept my commits small and atomic.
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📖 I have updated the README to account for my changes.
📑 I have documented new code with YARD.
🛣️ I have opened a PR to update the guides.
✅ I have added automated tests to cover my changes.
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solidusio / solidus_braintree