Load Stripe.JS library asynchronously

[DeanMarkTaylor]

As discussed in #2065 targeting performance issues of product and cart pages

Partially Fixes #2065

Intended to be separate from #2110 which is also addressing issues in #2065.

Changes proposed in this Pull Request:

Loads the Stripe.JS library asynchronously (with async attribute) as per the Stripe JS API recommendations
Targeting reduced "time to interactive" experience for users.

This is a work in-process sharing so others can review and test.

Known issues / incomplete

• WARNING: Zero Testing - I have just typed the code and not tested at-all.
• Low: There is a duplication of JavaScript code in assets/js/stripe.js and assets/js/stripe-pament-requiest.js. Unsure the value this there to break out duplicated code (extra HTTP request, async complexity). Probably a separate PR.
• Change log updates
• I have not generated the ".min.js" files (unsure of process)
• What (if any) JavaScript code can be pulled out and not be dependent on the knowledge that Stripe.JS being loaded. (separate PRs?)

Testing instructions

So far this has not been tested at-all.

Because this directly impacts the loading of the main Stripe.JS library the main things to check are:

• No console errors indicating problem loading "Stripe.JS"
• Card brand logo updates when inputting CC details. (Indicates some JS has loaded)
• Payment Request buttons load when expected on product page (again simply existing is positive indicator loading succeeded).
• Checkout process completes (actual communication with Stripe, the library must have loaded).

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)

[lkraav]

Can you perhaps amend with removing whitespace-only changes so PR would become easier to evaluate 🙏

[DeanMarkTaylor]

Can you perhaps amend with removing whitespace-only changes so PR would become easier to evaluate 🙏

Sure, this has been done.

The whitespace changes were automatically applied formatting for WordPress Coding Standards from the IDE.