Harden validator and prep for v2 release

[lucasros98]

Summary

Stabilizing the package ahead of a more robust release. This bundles bug fixes, a real type story, modernized tooling, and a restructured test suite.

Bug fixes

• Resolve 2-digit years against the current century rather than hardcoding + 2000 (a YY of 99 no longer becomes year 2099).
• Tighten the Finnish HETU regex — the previous [-|U-Y] character class accidentally allowed a literal |.
• Validate separator position for SE and DK input. '8601-013496' and similar misplaced hyphens are now rejected instead of silently stripped.
• Reject non-digit Norwegian input early instead of relying on downstream regex.
• Rename the misspelled teshLuhn -> testLuhn; the old name is kept as an alias for back-compat.

Types

• Ship index.d.ts with Validator, CountryCode, and ValidatorInput.
• Add test/types.test-d.ts and run tsc as part of npm test.

Tooling

• Migrate ESLint to flat config (eslint.config.js); bump to ESLint 9.
• Replace the single test workflow with a CI workflow that lints, tests on Node 18/20/22, and builds.
• Bump dev dependencies (Babel 7.29, AVA 6, ESLint 9, TypeScript 5.9).
• Raise engines.node to >=18.
• Add files allowlist and prepublishOnly script to package.json so publishes only ship dist/ + index.d.ts.

Tests

• Split the single test/test.js into per-country files plus a shared test/fixtures.js and a test/package.test.js smoke test for the public API.
• Renamed src/dannish-ssn.js -> src/danish-ssn.js (typo fix).

Docs

• Rewrote README.md with badges, an API table, accepted formats per country, and explicit notes on which checksums are validated (DK mod-11 was phased out in 2007 and is intentionally not checked).

Test plan

• npm run lint clean
• npm test — 295 tests pass, tsc passes
• CI matrix green on Node 18/20/22
• Verify npm pack only contains dist/ and index.d.ts