Collision fixes

[Geokureli]

For chosencharacters/squidBounties#6

Here are videos showing the changes.
https://imgur.com/a/9yXzCwk

summary:

• preserve momentum in collisions, fixing several bugs with mass. (Used this article as a guide and this to test)
• add a new collisionXDrag that takes an enum, to allow objects to stick to non-immovable objects.
• deprecate collisonXDrag (missing an "i" in collision)

breaking changes:

• the new collision will break old replays, so I added the FLX_4_LEGACY_COLLISION compiler flag

I'm still checking all the demos, but I'm pretty confident they will all work

[Geokureli]

Yeah all the demos are behaving nicely, and none of the snippets should be affected

[Geokureli]

Just noting: I tried modifying how separate() moves the colliding objects. I tried displacing inversely proportional to their mass, like so:

Object1.x -= overlap * mass2 / (mass1 + mass2);
Object2.x += overlap * mass1 / (mass1 + mass2);

this caused a lot of problems with stacking heavy objects on light objects, and pushing objects into other objects, so I decided against it.