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 be more inclined to remove the `@api` annotation from this.  This is implemented differently for different geometry types.  Internally, we want to have the ability to simplify to meet rendering needs.  From an API perspective, it makes more sense to have control over the method (which algorithm is used).  Since those goals are at odds, I don't think this is the best candidate for a stable API method.