Add Cartesian2.cross

[hpinkos]

No description provided.

[cesium-concierge]

Thanks for the pull request @hpinkos!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
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[IanLilleyT]

Code looks fine but if we use the name "cross product" there might need to be some explanation in the description like Computes the magnitude of the cross product that would result from implicitly setting the Z coordinate of the input vectors to 0. More elegant description welcome.

[hpinkos]

Works for me, thanks @IanLilleyT! Updated

[IanLilleyT]

Looks good, thanks @hpinkos !