modernize ui/learn

[schlawg]

1. Get rid of mithril, seems like it's just boilerplate.
2. Either get rid of chess.js dependency or convince this mythical ornicar person to put it in npm. some pnpm versions find github repo links in package.json offensive.
3. Investigate compatibility with chessground 9.0.4 (learn module links to a really old version and i'd like to know why).

[ornicar]

it uses a chessground fork to display stars and maybe other garbage on the board. I don't think we should make the normal chessground any heavier just for ui/learn.

But a newer fork could be used. Heck, maybe current chessground can do it on its own, idk.

[brollin]

A refactoring task that when finished, the user will never know? That's my jam. It was fun to see what people did with the coordinate trainer after that was snabbdomified.

At least from what I've seen, it does seem like most if not all of the required chessground functionality exists in the latest chessground. Namely allowing custom SVGs should get us pretty far I would think (🤞).