CreateTiledGround() function

Hello ! here is the TiledGround PR ! 
As I said, I did TS changings too. In fact you maybe don't need JS ones but they are here.
I hope TS is well written, I used the TypeScript Playground to make it correctly :)
Cheers
 
### Include
* VertexData.CreateTiledGround() function (TS & JS)
* Mesh.CreateTiledGround() function (TS & JS)

### Arguments of the function
* **xmin, zmin, xmax, zmax** : extent values of the ground
* **subdivisions** : the number of tiles
* **precision** : the number of subdivisions inside a tile.

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I chose to use GroundMesh but for the moment 
* optimize() function doesn't work with these new tiled grounds. (as expected)
* getHeightAtCoordinates() function works just as if we use CreateGround but I think there is a bug which need to be fixed. I'm already working on a new PR to fix it.