Make types used in the API part of the API

[ahocevar]

Currently there are a few types which are exposed as arguments or return types of exportable members. We either have to make these types exportable, or remove the symbols that expose them from the API. This is the list of types:

ol.TileRange
ol.format.Feature
ol.format.IGCZ
ol.format.WFS.FeatureCollectionMetadata
ol.format.WFS.TransactionResponse
ol.render.IVectorContext
ol.source.WMTSRequestEncoding
ol.style.IconAnchorUnits
ol.style.IconOrigin

This came up in #2479 (comment).

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[probins]

see #2509

[probins]

I think this one can be closed, as it should be covered by #2615