Add API to convert standard position (center/pitch/zoom/bearing) to FreeCameraOptions

Nobody has claimed this yet.

Currently there's no way to get a `FreeCameraOptions` object corresponding to a hypothetical map position defined in traditional terms (`center`, `zoom`, `pitch` and `bearing`), only for the current position.

I needed this to do a free-form interpolation between two cameras defined this way, and was able to achieve this through the hack of cloning the map transform, but there should be an easier API built-in:

```js
const fakeTransform = map.transform.clone();
fakeTransform.center = targetLngLat;
fakeTransform.zoom = targetZoom;
fakeTransform.bearing = targetBearing;
fakeTransform.pitch = targetPitch;
const targetCamera = fakeTransform.getFreeCameraOptions();
```

E.g. accepting options in `getFreeCameraOptions`:

```js
const camera = map.getFreeCameraOptions({bearing: 30, pitch: 10});
```

Start at the map.getFreeCameraOptions API and the map transform behavior shown in the issue, then trace how FreeCameraOptions is produced for the current position. Define the accepted standard position options and verify that the API returns the corresponding camera for center, zoom, pitch, and bearing, including the interpolation use case.

Written by the indexing model from the issue text.