Test the checkout code; publish images only for releases (#5)

- src/checkout/ (PayPal order request, pricing, VAT) and src/logging/format.ts
  hold the checkout's pure code, moved out of PayPalApi, the product service
  and config without a behaviour change.
- npm test runs node:test on the TypeScript sources (type stripping), with no
  dependencies: 21 tests, 13 pin today's output, 8 describe the correct
  behaviour for the known defects of libretech/mp#67
  and are marked todo.
- CI: the tests run in the image's base (node:22.14.0-alpine) for every pull
  request, push to main and release tag. The image is built for pull requests
  and built and published only for v* tags; main no longer publishes :main
  and :sha-* images.

Refs libretech/mp#71

Co-authored-by: Michael Czechowski <mail@librete.ch>

package.json

@@ -7,6 +7,7 @@
 		"build": "strapi build --debug",
 		"strapi": "strapi",
 		"lint:fix": "prettier --write \"{src,types,config}/**/*.{js,jsx,ts,tsx}\"",
+		"test": "node --experimental-strip-types --import ./tests/unit/support/register.mjs --test 'tests/unit/**/*.test.ts'",
 		"generate:types": "strapi ts:generate-types",
 		"import": "npm run strapi import -- -f database/export.tar.gz --force",
 		"export": "npm run strapi export -- --no-encrypt --file database/export_$(date +'%Y%m%d%H%M%S')",