[Proposal] Deprecate MotionBuilder.Preserve() and add it to MotionHandle

[AnnulusGames]

There is a MotionBuilder.Preserve() that keeps the builder unpooled, but this should be added to MotionHandle. Currently created motions are automatically discarded upon completion, but making this explicit would facilitate motion control.

[AnnulusGames]

It is possible to add this to MotionHandle while keeping the current API, but these could be confusing. While this is a breaking change, it should not be a problem if done at the same time as the currently planned transition to v2.