Moving from Projects to Milestones

[elalish]

I was finding the Github Projects page to be a bit heavyweight and we weren't using it that much, so I've decided to switch us to using Milestones instead, e.g. https://github.com/elalish/manifold/milestone/1

This way we can have one Milestone for each release, so it'll make it clearer how close we are and what I could especially use help with. You can see that we're most of the way to v2.2 - I just want to get our bindings updated and ideally publish to pypi at the same time. Please feel free to mention on any issue if you think it should be added to our current milestone (e.g. you plan to work on it) or removed (we shouldn't hold the release for it).