Add vehicle physics example

[willeastcott]

Adds a new vehicle physics example to the engine. It is based on the btRaycastVehicle API in ammo.js. The example creates a regular dynamic, compound collision body and augments it to be a raycast vehicle.

The example also adds some additional scripts:

• A tracking camera script that simply looks at a specified target entity as it moves around.
• A reset physics script which will reset all dynamic rigid bodies back to their start position/rotation on pressing the R key.

There is also an Editor project that uses the same scripts to implement a graphically more interesting scene:

It is available here:

https://playcanvas.com/project/643289/overview/vehicle-physics

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[mvaligursky]

nice one!