Added support for GLTF format, removed deprecated JSONLoader

[alexanderflink-zz]

Added support for GLTF format using three-gltf-loader. Tested with models from examples folder. Works with both .glb and .gltf. Needed to update three and @types/three to use the GLTF loader, and removed the JSONLoader since it was deprecated. ObjectLoader still works.

I think GLTF would be nicer to use than the three json format, since they removed the exporters and support for json format.

[fand]

Thanks @alexanderflink , this is great!

I tried GLTF files in examples/models/gltf/ directory by replacing PATH in examples/obj-mtl.frag,
And most files worked well 🙌

      // PATH: './models/gltf/AnimatedMorphSphere/glTF/AnimatedMorphSphere.gltf', // working???

      // PATH: './models/gltf/BoomBox/glTF/BoomBox.gltf',
      // PATH: './models/gltf/BoomBox/glTF-Binary/BoomBox.glb',
      // PATH: './models/gltf/BoomBox/glTF-dds/BoomBox.gltf', // not working
      // PATH: './models/gltf/BoomBox/glTF-pbrSpecularGlossiness/BoomBox.gltf',

      // PATH: './models/gltf/BotSkinned/glTF-MaterialsUnlit/Bot_Skinned.gltf',

      // PATH: './models/gltf/CesiumMan/glTF/CesiumMan.gltf',
      // PATH: './models/gltf/CesiumMan/glTF-Binary/CesiumMan.glb',
      // PATH: './models/gltf/CesiumMan/glTF-Draco/CesiumMan.gltf', // not working
      // PATH: './models/gltf/CesiumMan/glTF-Embedded/CesiumMan.gltf',

      // PATH: './models/gltf/CesiumMilkTruck/glTF/CesiumMilkTruck.gltf',
      // PATH: './models/gltf/CesiumMilkTruck/glTF-Binary/CesiumMilkTruck.glb',
      // PATH: './models/gltf/CesiumMilkTruck/glTF-Draco/CesiumMilkTruck.gltf', // not working
      // PATH: './models/gltf/CesiumMilkTruck/glTF-Embedded/CesiumMilkTruck.gltf',
      // PATH: './models/gltf/CesiumMilkTruck/glTF-MaterialsCommon/CesiumMilkTruck.gltf',
      // PATH: './models/gltf/CesiumMilkTruck/glTF-techniqueWebGL/CesiumMilkTruck.gltf',
      
      // PATH: './models/gltf/DamagedHelmet/glTF/DamagedHelmet.gltf',

      // PATH: './models/gltf/Duck/glTF/Duck.gltf',
      // PATH: './models/gltf/Duck/glTF/Duck.gltf',
      // PATH: './models/gltf/Duck/glTF-Binary/Duck.glb',
      // PATH: './models/gltf/Duck/glTF-dds/Duck.gltf', // not working
      // PATH: './models/gltf/Duck/glTF-pbrSpecularGlossiness/Duck.gltf',

      // PATH: './models/gltf/MetalRoughSpheres/glTF/MetalRoughSpheres.gltf',
      // PATH: './models/gltf/MetalRoughSpheres/glTF-Embedded/MetalRoughSpheres.gltf',

      // PATH: './models/gltf/Monster/glTF/Monster.gltf',
      // PATH: './models/gltf/Monster/glTF-Binary/Monster.glb',
      // PATH: './models/gltf/Monster/glTF-Draco/Monster.gltf', // not working
      // PATH: './models/gltf/Monster/glTF-Embedded/Monster.gltf',
      // PATH: './models/gltf/Monster/glTF-lights/Monster.gltf',

      // PATH: './models/gltf/OutlinedBox/OutlinedBox.gltf',

Some files not working, however they seem easy to fix.
we only need to load DRACO Loader and DDS Loader according to error messages .
I'll do that after merge, but if you do that i'll be appreciate it.

I'm gonna merge this PR soon. Could you merge master and resolve conflicts?