Update dependencies (TypeScript 3.9 → 5, Jest 24 → 29, @types/node, Babel) #173

Context

The dependency stack has drifted noticeably:

• TypeScript 3.9 → current 5.x. Likely the biggest pain point (strict null checks may shift, @types/* update, possible internal API tweaks).
• Node @types/node ^12 — outdated, doesn't cover modern APIs. Pin to whatever the minimum supported Node version is.
• Jest 24 → 29.x — lots of breaking changes between them, but jest-codemods is mature.
• Babel 7 stays as a major, but @babel/* packages have minor fixes.
• yargs, koa, koa-router, gettext-parser — check majors.

Good moment to bring the foundation up to date while attention to the project is warm.

Scope

• Pin the minimum supported Node version in engines (package.json) and in README. Required to pick the right @types/node.
• npm outdated + review each major separately. No bulk update.
• TypeScript 3.9 → 5.x (separate PR — potentially biggest impact)
• Jest 24 → 29 (separate PR; jest-codemods as a starting point)
• @types/node → current LTS
• Babel devDeps: bump @babel/* to latest 7.x patch/minor
• Other devDeps (Prettier, ESLint, husky, lint-staged) — bulk-bump separately
• Runtime deps (yargs, gettext-parser, koa, vue-sfc-parser, svelte) — review major breaking changes
• npm audit after everything
• Full test suite + smoke test on a real .po project

Out of scope

• Switching from yargs to commander/clipanion — separate discussion
• Switching from Babel to SWC/esbuild for extraction — architectural change, not a dep bump

Definition of Done

• npm outdated shows only intentionally pinned versions
• npm audit no high/critical
• npm test green
• Smoke test on a real project passes (extract → check → replace)
• engines.node fixed and reflected in README