* feat: initial implementation of contact manifold reduction
* feat: try bepu-like manifold reduction
* feat: simplification of the constraints counting and indexing logic
* feat: add concept of incremental islands with a single awake island
More islands manager fixes
* feat: start adding support for multiple awake islands
* feat: add more timings
* feat: implement incremental island split & merge
* chore: refactor islands manager into multiple files
* chore: refactor manifold reduction to its own file + add naive reduction method
* feat: add islands manager validation checks
* fix various bugs in the new islands system
* chore: remove redundant active_set_offset field
