[@mantine/core] Timeline: Allow manually overriding of active state

[Zegnat]

I was surprised that I was able to set active={true} (or just active) on a Timeline.Item to turn on the active presentation of an item, but if it was covered by Timeline’s active I was unable to set active={false} to turn off the active presentation.

Turns out falsy values for active were being ignored and always turned the element over to Timeline’s control. I propose to only let Timeline handle active state in case of undefined.

I am not too happy with the test, but it seems to confirm my fix. Not sure how deep Mantine usually goes with tests. Let me know if there is anything there you would want me to tweak!

Thanks for the awesome UI project ❤️

[rtivital]

Hi @Zegnat, sorry for long response, can you please resolve the conflicts?