Unify Lua API for world objects

Bill of materials to be done before starting this ticket:
* #1373

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After #1373, features units and projectiles will have unique IDs so (for example) `Spring.SetUnitPosition(unitID, xyz)` and `Spring.SetFeaturePosition(featureID, xyz)` will be redundant. There would be able to be a single `Spring.SetPosition(thingID, xyz)`.

Add unified interfaces where possible. Keep the old ones as aliases to the new ones.

Start by reading prerequisite issue #1373 to confirm how unique IDs for units and projectiles are represented. Review the existing Spring.SetUnitPosition and Spring.SetFeaturePosition Lua interfaces, then identify other redundant unit, feature, and projectile APIs that can share unified interfaces. Done means unified calls exist where possible and the old calls remain usable as aliases.

Written by the indexing model from the issue text.