Refactor the standard integration guide

[gregjopa]

The DevRel team has been working on improving the Standard Integration code snippets across developer.paypal.com. This PR includes many of these new changes.

Improvements include:

• Using the common naming of PAYPAL_CLIENT_ID and PAYPAL_CLIENT_SECRET for env variables.
• Using a single file for the Node.js server code.
• Teach negative testing with code comments for using the PayPal-Mock-Response request header for order creation and capture.
• Always try to relay PayPal API JSON responses and status codes back to the frontend.
• Death to alert() since it pauses the popup flow. Replacing this with appending UI messages to the button container.

There are some other changes I am less confident on and could use feedback on.

• What route names should we use for wrapping create order and capture order api calls?
• Whats the best way to pass the OrderID to the create order api wrapper? Pass it in the URL or in the post body?
• Should we recommend the Express static module or just return the html file directly from the file system?

Feedback welcome! Let us know what you like and don't like.

[wsbrunson]

looks amazing!

[wsbrunson]

one thing i would love to see is all of our examples using paypal-js

[gregjopa]

I'd be open to that. How do you think they should use paypal-js in a repo like this? Should we recommend a separate client-side js file that imports in paypal-js and then serve up a bundle via rollup or webpack? Or host it from paypalobjects.com and have folks download it from there?

[wsbrunson]

I think the later. I think ideally we'd have something that was as easy as stripe's: https://stripe.com/docs/libraries/stripejs-esmodule#web-stripejs-html

[wsbrunson]

I think with the former, we have to start talking about build tools and that always sucks

[gregjopa]

I hear ya on the burden of teaching build tools. I like offering both the CDN option and the ES Module like Stripe does. I'll take this paypal-js loader suggestion back to the team to get more feedback.

[jshawl]

Looks great! Left a few non-blocking comments

[jshawl]

💯 this tripped me up the other day thinking my-server was not an actual route...

[gregjopa]

Thanks for the feedback everyone! I applied some of the recommendations and I'm going to merge this PR.

@cnallam, fyi I changed two things with the codespaces integration for standard checkout:

1. There was a bug with the workspace path that is now updated.
2. The env variables have been renamed to PAYPAL_CLIENT_ID and PAYPAL_CLIENT_SECRET to better match the PayPal Auth docs and to help the developer know that these credentials are for PayPal. We plan to make the same change to the Advanced Integration in a follow up PR.

I've successfully tested the CodeSpaces integration off this feature branch. Here are some screenshots for reference: