Friction only accounts for dynamic friction

[Jondolf]

The Friction component stores coefficients of both dynamic and static friction. However, only dynamic (i.e. "sliding") friction is currently accounted for, and the static coefficient does nothing. This is a regression from bevy_xpbd (the predecessor of Avian).

I originally left out static friction because it was causing some serious instability issues, and most other engines like Rapier and Box2D also only consider dynamic friction. However, it could still be nice to have, and it could be that my earlier issues were actually related to #489, which is now fixed.

We should attempt to add back static friction, or if it turns out that it just doesn't work or is otherwise undesirable, we should remove the property from Friction.