`module: node20` introduced in Typescript 5.9 as a stable replacement
for `nodenext`:

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-9.html#support-for---module-node20

The recommended combination is with `moduleResolution: node16`:
microsoft/TypeScript#61805

That change implies relative imports to have extensions, however, it's
possible to change them directly to `.ts`, which will also reiterate and
resolve #2289 . It will enable running typescript natively by Node.js
without `tsx`. It may also come handy if I decide to publish to JSR.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- Chores
- Project build and compiler settings standardized; JSON imports made
explicit and some tooling switched to static JSON imports.
- Refactor
- Internal module references normalized to explicit TypeScript entries;
some runtime identifiers now derive from environment-provided build
metadata.
- Tests
- Test imports updated for explicit module resolution; migration tests
expanded with additional valid cases.
- Style
- New lint rules encourage .ts extensions for local imports and
discourage direct package.json imports.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->