Add get_planes_trie to FFI

[sffc]

trafficstars

Currently icu::collections::codepointtrie::get_planes_trie is not exported in FFI. It appears to be allowlisted along with all of the icu::collections crate; that filter pattern is probably too broad.

I think it makes sense to add this as a getter on CodePointMapData, even if the plane is not technically a Unicode property. Thoughts?

@Manishearth @ifeoluwa112

[Manishearth]

That seems reasonable!