Refresh (under-the-hood improvements)

No changes to external API.
Lots of internal updates and modernization:

• Upgrade to TypeScript 5.9 strict, still down-leveling to es2017.
  • Effectively obsoleting eslint, which has been removed.
• Most .then/.catch chains converted to async/await for better stack traces.
• Remove .js extensions from all TypeScript relative imports (crept in long ago).
• More tests.
• Binder interface deprecated.