* restrict main_menu system to only run in AppState::MenuRunning
* this is desired because hitting enter once we transition to AppState::AppRunning pressing enter will still trigger keycode.just_pressed in main_menu and set state to AppState::AppLoading and crash the game when it tries to create another scene, camera, etc...
* feat(examples): added basic_xpbd_physics example
* adapted code from rapier physics
* updated blend file & exported gltf files
* changes & tweaks to cargo files so the examples are scanned as they should be
* various minor tweaks
* refactor(examples): moved examples into more logic folders & changed to allow for per example dependencies
* chore(): updated docs, added docs, updated deps , cargo files etc
Shawn Workman
Mark Moissette