feat: Rework CharacterController based on Bepu's

[Eideren]

PR Details

The implementation for bepu's character controller was a bit wonky;

• The gravity property was mutated internally to prevent the body from sliding down slopes
• Moving surfaces would not carry the body along with them
• Moving past a slope would cause the body to fly off instead of continuing horizontally

Related Issue

None logged

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.

N.B.: CharacterControllers.cs, CharacterMotionConstraint.cs and CharacterMotionConstraint.tt are effectively copies from bepu's demo. Mostly left as is to make porting new features and fixes introduced over there easier.

[Eideren]

@xen2 looks like CI is broken, a bunch of unrelated nuget errors ?

[mihai-ene-public]

hi, since you're doing a rework; could be possible that either in this PR or another to have an Enabled property on Collidables? related to this: #2923
:)

[Eideren]

There are thousands of things that I want to tackle, and I already don't have time to take care of those. I'm sure you'll figure this out

[xen2]

@xen2 looks like CI is broken, a bunch of unrelated nuget errors ?

Sorry about that, on it!