Improve payment form handling and start using `@stripe/stripe-js` module

[elia]

Summary

Extracted from #249

Use the @stripe/stripe-js module with importmap, as partially described in the Stripe doc.

The <script> tag is added to every page to support fraud detection as suggested in the stripe-js repo. We opted for using the script tag instead of relying on the implicit side-effect because the browser will be able to start loading it earlier if it's directly in the HTML.

https://github.com/stripe/stripe-js#ensuring-stripejs-is-available-everywhere

Checklist

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The following are mandatory for all PRs:

• [ ] I have written a thorough PR description.
• [ ] I have kept my commits small and atomic.
• [ ] I have used clear, explanatory commit messages.

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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.
• 📸 I have attached screenshots to demo visual changes.

[codecov[bot]]

Codecov Report

Merging #255 (07723f1) into master (682579e) will decrease coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #255      +/-   ##
==========================================
- Coverage   99.79%   99.79%   -0.01%     
==========================================
  Files          25       25              
  Lines         481      480       -1     
==========================================
- Hits          480      479       -1     
  Misses          1        1              

Impacted Files	Coverage Δ
...lates/app/views/checkouts/payment/_stripe.html.erb	100.00% <ø> (ø)

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