Add @api stable annotations for ol.Feature and the ol.geom namespace

This PR adds `@api stable` annotations to constructors and methods for `ol.Feature` and the `ol.geom` package.

Please review.
I'd rename this to `ol.geom.PointCoordinate` first (and same with all `ol.geom.Raw*` below).

And really, `ol.Coordinate` would make for fewer clicks when people are reading the API docs.  Instead of `ol.geom.RawMultipolygon` we'd have to write `Array.<Array.<Array.<ol.Coordinate>>>` in the source.  But I don't think these additional types really bring us much benefit.