feat(rapier_testbed): autosave testbed configuration + add support for per-example settings (#799)

* feat(rapier_testbed): autosave testbed configuration + add support for per-example parameters

* feat(rapier_testbed): also save the camera position

* feat(rapier_testbed): improve ergonomics of example-specific settings

* chore: cargo fmt

* chore(rapier_testbed): small UI ergonomics improvement

* chore(rapier_testbed): clippy fixes

crates/rapier_testbed2d/Cargo.toml

@@ -61,6 +61,8 @@ bevy_pbr = "0.15"
 bevy_sprite = "0.15"
 profiling = "1.0"
 puffin_egui = { version = "0.29", optional = true }
+serde = { version = "1.0.215", features = ["derive"] }
+serde_json = "1"
 
 # Dependencies for native only.
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
@@ -75,6 +77,7 @@ bevy = { version = "0.15", default-features = false, features = [
     "bevy_render",
     "bevy_pbr",
     "bevy_gizmos",
+    "serialize"
 ] }
 
 # Dependencies for WASM only.