Add `Spring.SetUnitPositionXZ`

See #1363 - currently SetPosition accepts two separate sets of args:
* either `x, y, z`,
* OR `x, z, bool surface vs underwater`

Split the XZ variant away into its own function, say `Spring.SetUnitPositionXZ`. Keep the functionality in regular `SetUnitPosition` for a while so that games don't immediately break, but don't mention the legacy args set it in the docs.

Start by tracing the existing SetUnitPosition implementation and read the related issue #1363 to understand the current argument variants. Done means a separate Spring.SetUnitPositionXZ function exists, the existing variants continue working temporarily, and the documentation does not describe the legacy XZ arguments.

Written by the indexing model from the issue text.