Add @api stable annotations to ol.events.condition

This PR adds stability markers to `ol.events.ConditionType` and some of the `ol.events.condition` built-in condition functions.

This also adds stability markers to properties of `ol.MapBrowserEvent` and `ol.MapEvent`. But note that we will still be able to rename, for example, `ol.MapBrowserEvent` to `ol.event.MapBrowser` without breaking the backward compatibility.

Please review.