fix(MessagesList): rename method to pollNewMessages

[Antreesy]

☑️ Resolves

• Non-breaking pre-requisite to feat(MessagesList) - history mode for old messages #10084
  • I'm giving up and will implement it by parts 👀
• rename getNewMessages to pollNewMessages (to avoid confusion - get*Messages should not poll)
• drop lookForNewMessages method as redundant
  • _isBeingDestroyed, _isDestroyed are internal Vue flags, different in Vue3, and not needed here
• refactor fetchMessages to support both direction of lookIntoFuture (to make fetching the chunk of new messages possible)
• refactor tests

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

No visual changes

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Not risky to browser differences / client
• ⛑️ Tests are included or not possible