Feature normal at point

[ThierryBerger]

Some shapes are missing ``

• halfspace -> todo?
• triangle (2d) -> attempt 🤔 triangle has no edge?
• trimesh (2d) -> not attempted 🤔 -> get all neighbour vertices and compute normal ? sounds convoluted
• Cone -> attempt ; math can probably be simplified: see https://github.com/user-attachments/assets/f068cd71-35dd-4f00-b400-0e2c175c850e
• Cylinder -> done ; check if faces are ok (to challenge with cone too)
• heightfield -> todo -> ⚠️ more difficult
• polyline -> todo -> ⚠️ more difficult
• Voxels -> ⚠️ more difficult
• compound -> impossible? we'd need to know which shape we're targetting the feature on (more complex featureId, out of scope for this PR)
• roundshape -> what whould that mean? delegate to inner shape? todo?
• identifying features is complex due to 2 ways of doing that (ncollide/parry)
• TODO: lowprio: return err<notimplemented> or no such feature
  • This leads to controversial api changes so I'd like to keep this PR simple

Cone implementation can probably be simplified from:

[ThierryBerger]

the concept of Feature and FeatureId is a bit complex and may need a more hoolistic approach, I'm pausing my work on it.