Add independent Rust engine, deterministic mod API and asset pipeline

examples/mods/aurora/scripts/drone.rhai

@@ -0,0 +1,16 @@
+fn on_spawn(event, state, random) {
+    #{ commands: [], state: state }
+}
+
+fn on_tick(event, state, random) {
+    // on_tick may be queued on the same step as on_death; do not command a retired entity.
+    if event.health <= 0 { return #{ commands: [], state: state }; }
+    let sideways = if (event.tick / 24) % 2 == 0 { 1 } else { -1 };
+    #{ commands: [#{ op: "velocity", entity: event.entity, vx: sideways, vy: 1 }], state: state }
+}
+
+fn on_death(event, state, random) {
+    let destroyed = if state.contains("destroyed") { state.destroyed + 1 } else { 1 };
+    #{ commands: [#{ op: "objective", key: "aurora:destroyed", value: destroyed }],
+       state: #{ destroyed: destroyed } }
+}