Convert all content from Kenney's 3D assets to use with Scene3D

[aglitchman]

Describe the content you'd like
I'd like to use 3D assets from Kenney: https://kenney.nl/assets?q=3d
They are available for free and can be used commercially.

The idea is to:

1. Mass-convert all the assets from Kenney and put them into a separate repository (defold-scene3d-kenney). The repo will be huge.
2. Wrap all models into game objects that will have the same components as Scene3D's icon, i.e. mesh, collisionobject, script.
3. Render previews for all models and put them in repo's README to know what we have.

I think it'll be fun to be able to put any Kenney's 3D put into your scene, modify collision, material, animation and make a game right from the Defold IDE in a "zero-coding" manner.

Describe alternatives you've considered (optional)
It's time-consuming to dig in Kenney's asset library and put them in your scene because you do that one by one.

[aglitchman]

I think it's easy to do with Houdini as I did that for the icons. Let's do that after #19