[Feat] setZoom method

[lucadalli]

I think a setZoom method alongside properties minZoom and maxZoom which allow zoom to be set programmatically would be a good addition.

This would enable zoom sliders like this one:

[elalish]

We already have this, but it's set by two parameters: camera radius and fieldOfView: https://modelviewer.dev/docs/#entrydocs-stagingandcameras-attributes-cameraOrbit and https://modelviewer.dev/docs/#entrydocs-stagingandcameras-attributes-fieldOfView

[lucadalli]

Manipulating camera-radius as a percentage value seems to be sufficient to achieve the desired zoom in and out effect. Thanks.