support xpbd collider gen

[shnewto]

context

• this crate's main function is grabbing the points that make up the outline of a sprite from transparency and creating a physics engine collider. There's no reason that needs to be a rapier collider!
• bevy_xpbd_2d is my first target for something beyond bevy_rapier2d
• when this PR or equivalent work is merged to support more than just rapier2d colliders, I'll rename this repo and produce a new crate that can do both
• more thoughts in bevy_xpbd collider gen? #7

approach

• i'm wrestling a bit with how to gate these engines as features, i.e. one file that has #[cfg(feature(...))] or multiple files with some duplication and minimal need for #[cfg(feature(...))]. at the moment i'm favoring the latter, it feels like it'll be easier to work with a specific engine in mind if you don't have to worry about bumping something for another one. It will change the API but I think it'll be basically a new crate so I'm not so worried about breaking changes, just don't want it too feel to tangled to use this stuff, it's supposed to make life a bit easier.
• atm i feel like just favoring support for xpbd over making all the right design decisions, anything left to be smoothed out can be in later releases

remaining questions/issues to resolve before merging

• i haven't figured out how to do native heightfields in xpbd
• i didn't spot convex polyline colliders in xpbd, are they known as something else or are they just not supported
• i fumbled using polyline colliders in the xpbd example so switched to convex hull
• re: the strikeouts above, I was able to use Collider::from(parry::SharedShape) to support convex polyline colliders and heightfields with xpbd_2d

[shnewto]

@CrockAgile if you're able to take a look at the cfg feature stuff i'm trying to work out, I'd be glad! 😄 🙏

[shnewto]

@Jondolf if you have any opinions on any of this goes down, i'd be glad to hear. would also be glad for pointers on using xpbd heightfeilds 😅

[shnewto]

okay all, I'm going to pull the trigger. I'll make some help wanted issues for helping me make the examples a little more engine-centric once things have been renamed and published