refactor: move from bevy_heron to bevy_rapier2d

[CashWasabi]

Summary

The refactor consists of basically two things:

• show use of bevy_rapier2d in platformer example
• add bevy_rapier2d in cargo file and remove heron

For the most part I tried to not change too much of the platformer example. There we're some minor things though that came up whith which I had to struggle a bit.

Notable Details

• rename CollisionShape -> Collider
• rename Velocity.linear -> Velocity.linvel
• create SensorBundle since there is no RigidBody::Sensor in rapier2d
• set player friction to 0.0 because the collider kept sticking to the wall when moving against it in air
• add RapierDebugRenderPlugin to showcase colliders with gizmos
• pause_physics_during_load uses RapierConfiguration now
• add ExternalForce to ColliderBundle for jumps

Issues

I noticed that my fix for pause_physics_during_load might not work as excepted. Sometimes when building I have to press R to reload the level when I want to have the GroundSensor working. This might be a loading issue but I didn't find the correct solution yet.

[CashWasabi]

I added the changes you last mentioned and ran cargo fmt on the codebase. I also squashed the commit since the last two commits we're just small fixes with no real information.

[Trouv]

I also squashed the commit since the last two commits we're just small fixes with no real information.

In the future, there's no need to do this for this repo since I squash commits on merge (as of 0.4) anyway. Actually, having the smaller commits before merge makes it easier to see what has changed since the last review

[janos-r]

I was working on the same update (not noticing this PR exists). In my case, I found it necessary to use .normalize_or_zero() instead of .normalize() because the first mob (spawning closest to the player), when returning, got to the exact same spot from where he was starting, creating his vector zero, having .normalize panic. I wonder why It doesn't happen here (that he doesn't return to the exact spot).