feat: particle options

packages/entity/entity.synchronized.js

@@ -435,4 +435,8 @@ export default class Entity extends decorate(Resource) {
     };
   }
 
+  traitInstance(type) {
+    return this._traits[type];
+  }
+
 }

packages/physics/traits/emitter.trait.js

@@ -41,6 +41,10 @@ export default class Emitter extends decorate(Trait) {
         args: [
           ['key', {
             type: 'string',
+            options: (entity) => (
+              Object.keys(entity.traitInstance('emitter').particles)
+                .reduce((r, key) => ({...r, [key]: key}), {})
+            ),
           }],
           ['json', {
             type: 'object',