feat(body): add isTrigger option

[marcofugaro]

Originally proposed here schteppe#146 and coded in this PR schteppe#424.

Trigger allow you to detect if a body has entered a certain zone. They can let you run code when the body either entered or exited the trigger. They exist in other game engines such as Unity or Unreal.

You define a trigger by passing using the isTrigger option.

const boxShape = new CANNON.Box(new CANNON.Vec3(2, 2, 5))
const triggerBody = new CANNON.Body({ isTrigger: true })
triggerBody.addShape(boxShape)

[marcofugaro]

we should probably emit endContact on the body as well