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>

tests/unit/support/register.mjs

@@ -0,0 +1,6 @@
+// Loaded by `npm test` through `node --import`. Node runs the .ts sources directly (type stripping), but unlike tsc it does not
+// resolve extensionless relative imports such as `import { vatIncludedDecimal } from "./vat"` in src/checkout/paypal-order.ts.
+// The hook below retries such an import from a .ts file with ".ts" appended. Test files import sources with explicit .ts paths.
+import { register } from "node:module";
+
+register("./resolve-ts.mjs", import.meta.url);