OrbitControls: add getDistance()

[marcofugaro]

Related issue: remake of #21162

Description

Add OrbitControls.getDistance() which exposes spherical.radius.

This is useful for example if you want to enable zooming in but disable zooming out from the start position:

const controls = new OrbitControls(camera, renderer.domElement);
const currentDistance = controls.getDistance();
controls.maxDistance = currentDistance;

[WestLangley]

I do not think these internals should be exposed, but at a minimum, methods must return the correct answer. In this case, that includes orthographic camera.

[WestLangley]

This PR does not produce the correct answer for orthographic camera.

Why not compute the distance to the target and return the result?

[marcofugaro]

@WestLangley I treated getDistance exactly like minDistance and maxDistance. They are not available for orthographic camera as well. Should those be changed as well in your opinion?

[WestLangley]

@marcofugaro No, I would just focus on this PR. If you compute the actual distance, the returned value will be correct.

I personally prefer no method to one that can return the incorrect answer.

[marcofugaro]

@WestLangley done, how does it look now?

[marcofugaro]

All done!

[mrdoob]

Thanks!