[@mantine/form] use-form: Make functions referentially stable

[mattrunyon]

Fixes #1196, #1193

This makes all functions returned by useForm stable. I used the eslint react-hooks plugin for their exhaustive dependencies rule while converting these. It helped catch everywhere that the state needed to be switched to using the ref value within the useCallback uses

[rtivital]

I haven't checked how it works, but if tests are passing, then it is 👍

[mattrunyon]

Made the changes you suggested (and also made another helper hook to track the state of a prop in a ref).

I also added a unit test for checking the functions are stable. Tested it failed when I removed useCallback from one of functions returned by the hook

[rtivital]

Thanks!

[rtivital]

I've merged it without checking how it works in real demos (stupid of me) and it turns out that it does not work at all – all demos in broken (values updates are random). I've reverted the changes, so if you want, you can open another PR for this.