Make PreparePlugin configurable

[Rigidity]

Objective

It's important to be able to disable transform->position and position->transform syncing in the PreparePlugin so that positions don't get messed up if using an alternative coordinate system, such as a floating origin. This PR makes PreparePlugin more configurable and exposes a more detailed PrepareSet to make external customization easier to manage.

Solution

Adds position_to_transform and transform_to_position fields to a new PrepareConfig, and exposes PrepareSet to the public so that you can add your own systems there without leaking too many implementation details.

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Changelog

Added

• Added PrepareConfig, based off of SyncConfig.

Changed

• Split up PrepareSet into individual groups.
• Refactored the system scheduling in PreparePlugin.
• Made init_transforms respect PrepareConfig.

[Jondolf]

Reviewed this finally; The new structure looks good, and I like the documentation a lot. This adds some useful flexibility and configurability. Thanks!