feat: add gemini to explain a move

[rivalarya]

No description provided.

[jthunderbird]

I did test this out and have some findings (not sure if @GuillaumeSD plans to implement this but it has potential amazing potential IMO). Findings:

• at least deploying this via docker-compose, I have a UI issue in that when I add my API key and hit save, it does nothing. When I click back to the explanation tab, it still has the same error. If I close the gemini window and reopen, same thing though it does appear to store the key. If I refresh the page then click the icon, it loads and gives me my gemini explanation
  • I would suggest obviously fixing the bug, but also maybe a little green "Saved!" or something once you click save instead of nothing
  • Would love to see the Explanation and Setting tabs while on them (they disappear and just show the tab you are not on)
• The prompt is a little off with the bestMove variable - it explains the best move would have been be what the next best move is for the other player (in my example, guy hung his Q by moving his pinned N so it says best move would have been my B taking his Q which is obviously the next best move, but not what the hanging players best move would have been)
• Maybe fixing the prompting/variables will fix this issue as well, but right now it also says the best move is for Q to take my B (which obviously was not possible since his N was in the way and now its my turn)
• Very minor and maybe should be ignored, but an update on the README for this new feature in the features section should probably exist

I still like the basic intro sentence and plan on testing it against more elaborate moves (see how it does when the mistake is because of like a 5 move mate or something). I am not affiliated with chesskit other than an avid user and lover of it, but am happy to help test more if @GuillaumeSD is seriously considering this feature (I vote yes).