feat: add timezone utilities for Workers

[rdlabo]

Summary

• Add an unpublished @rdlabo/workers-timezone package with a module-instance default timezone.
• Support IANA zones, DST gaps and overlaps, unusual offsets, local-date conversion, day boundaries, and calendar arithmetic.
• Preserve the existing workers-hono-kit/business-time API names and Asia/Tokyo default for migration compatibility.
• Add Node 20/22 CI and a packed-tarball runtime/type consumer smoke test.

Safety

• private: true prevents npm publication at this stage.
• No npm publish or release workflow is included.

Validation

• npm run format:check
• npm run typecheck
• npm test (10 tests)
• npm run build
• npm run test:package

Review loop

• Upper-management review: passed after fixing compatibility, date validation, year 0099, and output-range issues.
• Independent acceptance review: passed after tarball consumer, world-timezone, DST, and invalid-input verification.

Follow-up before npm publication

• Make this package the canonical implementation.
• Change @rdlabo/workers-hono-kit/business-time to a deprecated re-export and remove duplicate logic.

[rdlabo]

Superseded by the monorepo implementation in rdlabo-dev/workers-hono-kit#43.