Add a way to specify the ClusterMode of a previewed cluster

[cormacrelf]

To be clear the cite-level ones, e.g. { id: "jones2006", prefix: "see their other work", mode: "SuppressAuthor" } currently work from the previewCitationCluster API, simply add a mode flag to any cite that you're passing in. It is only the cluster level flags { id: "cluster-one", cites: [...], mode: "Composite" } that aren't yet exposed there. To get that, I'll have to add a parameter to let you supply the preview cluster's cluster mode.

Originally posted by @cormacrelf in #117 (comment)

[cormacrelf]

Added in #121, wasm will require a breaking change too, I'll do that separately.