Feat/kepler

[thombruce]

closes #28, closes #48

_Partially based on https://github.com/atbentley/bevy_mod_orbits/ with some needed modifications. Most notably: Z values are now Y values, as we're dealing exclusively in the 2-dimensional domain._

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Internal use. Do not delete.

• [ ] Tests passing
• [ ] Coverage sufficient
• [ ] Manual review
• [ ] No A11y regression
• [ ] Translations provided or not needed

[thombruce]

TODO

• [x] Add mass to Orbitable component or create separate Mass component (it's probably useful for non-orbitables too, maybe in the pull to planet effect)
• [x] (Probably) remove orbital effects for bodies without a parent (think "ghost planets" that are free-floating in space)
• [x] Reverse orbits (they should be counter-clockwise)[^1]

[^1]: This should be true for our home system, but it isn't necessarily true of others nor is it necessarily a default. What is true is that most planets in a system should orbit in the same direction (the direction of angular momentum when the system formed, and the same direction as the angular momentum of the parent star). Some planets might orbit retrograde, but it should be rare and it should imply a catastrophically large collision or other enormous force.